"IO" -> "System".

Get rid of the separate opt-in IO class and replace it with a core
System class.

- Remove wren_io.c, wren_io.h, and io.wren.
- Remove the flags that disable it.
- Remove the overloads for print() with different arity. (It was an
  experiment, but I don't think it's that useful.)
- Remove IO.read(). That will reappear using libuv in the CLI at some
  point.
- Remove IO.time. Doesn't seem to have been used.
- Update all of the tests, docs, etc.

I'm sorry for all the breakage this causes, but I think "System" is a
better name for this class (it makes it natural to add things like
"System.gc()") and frees up "IO" for referring to the CLI's IO module.

test/core/string_byte_sequence/count.wren

@@ -1,15 +1,15 @@
 // Simple.
-IO.print("".bytes.count) // expect: 0
-IO.print("123".bytes.count) // expect: 3
+System.print("".bytes.count) // expect: 0
+System.print("123".bytes.count) // expect: 3
 
 // UTF-8.
 // Bytes:
 //        123456789
 // Chars: sø mé ஃ
-IO.print("søméஃ".bytes.count) // expect: 9
+System.print("søméஃ".bytes.count) // expect: 9
 
 // Null bytes.
-IO.print("\0\0\0".bytes.count) // expect: 3
+System.print("\0\0\0".bytes.count) // expect: 3
 
 // Invalid UTF-8.
-IO.print("\xef\xf7".bytes.count) // expect: 2
+System.print("\xef\xf7".bytes.count) // expect: 2

test/core/string_byte_sequence/iterate.wren

@@ -3,19 +3,19 @@
 // Chars: sø mé ஃ
 var bytes = "søméஃ".bytes
 
-IO.print(bytes.iterate(null)) // expect: 0
-IO.print("".bytes.iterate(null)) // expect: false
+System.print(bytes.iterate(null)) // expect: 0
+System.print("".bytes.iterate(null)) // expect: false
 
-IO.print(bytes.iterate(0)) // expect: 1
-IO.print(bytes.iterate(1)) // expect: 2
-IO.print(bytes.iterate(2)) // expect: 3
-IO.print(bytes.iterate(3)) // expect: 4
-IO.print(bytes.iterate(4)) // expect: 5
-IO.print(bytes.iterate(5)) // expect: 6
-IO.print(bytes.iterate(6)) // expect: 7
-IO.print(bytes.iterate(7)) // expect: 8
-IO.print(bytes.iterate(8)) // expect: false
+System.print(bytes.iterate(0)) // expect: 1
+System.print(bytes.iterate(1)) // expect: 2
+System.print(bytes.iterate(2)) // expect: 3
+System.print(bytes.iterate(3)) // expect: 4
+System.print(bytes.iterate(4)) // expect: 5
+System.print(bytes.iterate(5)) // expect: 6
+System.print(bytes.iterate(6)) // expect: 7
+System.print(bytes.iterate(7)) // expect: 8
+System.print(bytes.iterate(8)) // expect: false
 
 // Out of bounds.
-IO.print(bytes.iterate(123)) // expect: false
-IO.print(bytes.iterate(-1)) // expect: false
+System.print(bytes.iterate(123)) // expect: false
+System.print(bytes.iterate(-1)) // expect: false

test/core/string_byte_sequence/iterator_value.wren

@@ -3,22 +3,22 @@
 // Chars: sø mé ஃ
 var bytes = "søméஃ".bytes
 
-IO.print(bytes.iteratorValue(0)) // expect: 115
-IO.print(bytes.iteratorValue(1)) // expect: 195
-IO.print(bytes.iteratorValue(2)) // expect: 184
-IO.print(bytes.iteratorValue(3)) // expect: 109
-IO.print(bytes.iteratorValue(4)) // expect: 195
-IO.print(bytes.iteratorValue(5)) // expect: 169
-IO.print(bytes.iteratorValue(6)) // expect: 224
-IO.print(bytes.iteratorValue(7)) // expect: 174
-IO.print(bytes.iteratorValue(8)) // expect: 131
+System.print(bytes.iteratorValue(0)) // expect: 115
+System.print(bytes.iteratorValue(1)) // expect: 195
+System.print(bytes.iteratorValue(2)) // expect: 184
+System.print(bytes.iteratorValue(3)) // expect: 109
+System.print(bytes.iteratorValue(4)) // expect: 195
+System.print(bytes.iteratorValue(5)) // expect: 169
+System.print(bytes.iteratorValue(6)) // expect: 224
+System.print(bytes.iteratorValue(7)) // expect: 174
+System.print(bytes.iteratorValue(8)) // expect: 131
 
-IO.print(bytes.iteratorValue(-9)) // expect: 115
-IO.print(bytes.iteratorValue(-8)) // expect: 195
-IO.print(bytes.iteratorValue(-7)) // expect: 184
-IO.print(bytes.iteratorValue(-6)) // expect: 109
-IO.print(bytes.iteratorValue(-5)) // expect: 195
-IO.print(bytes.iteratorValue(-4)) // expect: 169
-IO.print(bytes.iteratorValue(-3)) // expect: 224
-IO.print(bytes.iteratorValue(-2)) // expect: 174
-IO.print(bytes.iteratorValue(-1)) // expect: 131
+System.print(bytes.iteratorValue(-9)) // expect: 115
+System.print(bytes.iteratorValue(-8)) // expect: 195
+System.print(bytes.iteratorValue(-7)) // expect: 184
+System.print(bytes.iteratorValue(-6)) // expect: 109
+System.print(bytes.iteratorValue(-5)) // expect: 195
+System.print(bytes.iteratorValue(-4)) // expect: 169
+System.print(bytes.iteratorValue(-3)) // expect: 224
+System.print(bytes.iteratorValue(-2)) // expect: 174
+System.print(bytes.iteratorValue(-1)) // expect: 131

test/core/string_byte_sequence/subscript.wren

@@ -3,22 +3,22 @@
 // Chars: sø mé ஃ
 var bytes = "søméஃ".bytes
 
-IO.print(bytes[0]) // expect: 115
-IO.print(bytes[1]) // expect: 195
-IO.print(bytes[2]) // expect: 184
-IO.print(bytes[3]) // expect: 109
-IO.print(bytes[4]) // expect: 195
-IO.print(bytes[5]) // expect: 169
-IO.print(bytes[6]) // expect: 224
-IO.print(bytes[7]) // expect: 174
-IO.print(bytes[8]) // expect: 131
+System.print(bytes[0]) // expect: 115
+System.print(bytes[1]) // expect: 195
+System.print(bytes[2]) // expect: 184
+System.print(bytes[3]) // expect: 109
+System.print(bytes[4]) // expect: 195
+System.print(bytes[5]) // expect: 169
+System.print(bytes[6]) // expect: 224
+System.print(bytes[7]) // expect: 174
+System.print(bytes[8]) // expect: 131
 
-IO.print(bytes[-9]) // expect: 115
-IO.print(bytes[-8]) // expect: 195
-IO.print(bytes[-7]) // expect: 184
-IO.print(bytes[-6]) // expect: 109
-IO.print(bytes[-5]) // expect: 195
-IO.print(bytes[-4]) // expect: 169
-IO.print(bytes[-3]) // expect: 224
-IO.print(bytes[-2]) // expect: 174
-IO.print(bytes[-1]) // expect: 131
+System.print(bytes[-9]) // expect: 115
+System.print(bytes[-8]) // expect: 195
+System.print(bytes[-7]) // expect: 184
+System.print(bytes[-6]) // expect: 109
+System.print(bytes[-5]) // expect: 195
+System.print(bytes[-4]) // expect: 169
+System.print(bytes[-3]) // expect: 224
+System.print(bytes[-2]) // expect: 174
+System.print(bytes[-1]) // expect: 131