"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/language/function/empty_body.wren

@@ -1,2 +1,2 @@
 var f = Fn.new {}
-IO.print(f.call()) // expect: null
+System.print(f.call()) // expect: null

test/language/function/newline_body.wren

@@ -1,4 +1,4 @@
 var f = Fn.new {
   // Hi.
 }
-IO.print(f.call()) // expect: null
+System.print(f.call()) // expect: null

test/language/function/newline_in_expression_block.wren

@@ -1,2 +1,2 @@
-Fn.new { IO.print("ok") // expect error
+Fn.new { System.print("ok") // expect error
 }.call()

test/language/function/no_newline_before_close.wren

@@ -1,2 +1,2 @@
 Fn.new {
-  IO.print("ok") } // expect error
+  System.print("ok") } // expect error

test/language/function/parameters.wren

@@ -1,50 +1,50 @@
 var f0 = Fn.new { 0 }
-IO.print(f0.call()) // expect: 0
+System.print(f0.call()) // expect: 0
 
 var f1 = Fn.new {|a| a }
-IO.print(f1.call(1)) // expect: 1
+System.print(f1.call(1)) // expect: 1
 
 var f2 = Fn.new {|a, b| a + b }
-IO.print(f2.call(1, 2)) // expect: 3
+System.print(f2.call(1, 2)) // expect: 3
 
 var f3 = Fn.new {|a, b, c| a + b + c }
-IO.print(f3.call(1, 2, 3)) // expect: 6
+System.print(f3.call(1, 2, 3)) // expect: 6
 
 var f4 = Fn.new {|a, b, c, d| a + b + c + d }
-IO.print(f4.call(1, 2, 3, 4)) // expect: 10
+System.print(f4.call(1, 2, 3, 4)) // expect: 10
 
 var f5 = Fn.new {|a, b, c, d, e| a + b + c + d + e }
-IO.print(f5.call(1, 2, 3, 4, 5)) // expect: 15
+System.print(f5.call(1, 2, 3, 4, 5)) // expect: 15
 
 var f6 = Fn.new {|a, b, c, d, e, f| a + b + c + d + e + f }
-IO.print(f6.call(1, 2, 3, 4, 5, 6)) // expect: 21
+System.print(f6.call(1, 2, 3, 4, 5, 6)) // expect: 21
 
 var f7 = Fn.new {|a, b, c, d, e, f, g| a + b + c + d + e + f + g }
-IO.print(f7.call(1, 2, 3, 4, 5, 6, 7)) // expect: 28
+System.print(f7.call(1, 2, 3, 4, 5, 6, 7)) // expect: 28
 
 var f8 = Fn.new {|a, b, c, d, e, f, g, h| a + b + c + d + e + f + g + h }
-IO.print(f8.call(1, 2, 3, 4, 5, 6, 7, 8)) // expect: 36
+System.print(f8.call(1, 2, 3, 4, 5, 6, 7, 8)) // expect: 36
 
 var f9 = Fn.new {|a, b, c, d, e, f, g, h, i| a + b + c + d + e + f + g + h + i }
-IO.print(f9.call(1, 2, 3, 4, 5, 6, 7, 8, 9)) // expect: 45
+System.print(f9.call(1, 2, 3, 4, 5, 6, 7, 8, 9)) // expect: 45
 
 var f10 = Fn.new {|a, b, c, d, e, f, g, h, i, j| a + b + c + d + e + f + g + h + i + j }
-IO.print(f10.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)) // expect: 55
+System.print(f10.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)) // expect: 55
 
 var f11 = Fn.new {|a, b, c, d, e, f, g, h, i, j, k| a + b + c + d + e + f + g + h + i + j + k }
-IO.print(f11.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)) // expect: 66
+System.print(f11.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)) // expect: 66
 
 var f12 = Fn.new {|a, b, c, d, e, f, g, h, i, j, k, l| a + b + c + d + e + f + g + h + i + j + k + l }
-IO.print(f12.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)) // expect: 78
+System.print(f12.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)) // expect: 78
 
 var f13 = Fn.new {|a, b, c, d, e, f, g, h, i, j, k, l, m| a + b + c + d + e + f + g + h + i + j + k + l + m }
-IO.print(f13.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)) // expect: 91
+System.print(f13.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)) // expect: 91
 
 var f14 = Fn.new {|a, b, c, d, e, f, g, h, i, j, k, l, m, n| a + b + c + d + e + f + g + h + i + j + k + l + m + n }
-IO.print(f14.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)) // expect: 105
+System.print(f14.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)) // expect: 105
 
 var f15 = Fn.new {|a, b, c, d, e, f, g, h, i, j, k, l, m, n, o| a + b + c + d + e + f + g + h + i + j + k + l + m + n + o }
-IO.print(f15.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)) // expect: 120
+System.print(f15.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)) // expect: 120
 
 var f16 = Fn.new {|a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p| a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p }
-IO.print(f16.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)) // expect: 136
+System.print(f16.call(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)) // expect: 136

test/language/function/syntax.wren

@@ -1,25 +1,25 @@
 // Single expression body.
-Fn.new { IO.print("ok") }.call() // expect: ok
+Fn.new { System.print("ok") }.call() // expect: ok
 
 // Curly body.
 Fn.new {
-  IO.print("ok") // expect: ok
+  System.print("ok") // expect: ok
 }.call()
 
 // Multiple statements.
 Fn.new {
-  IO.print("1") // expect: 1
-  IO.print("2") // expect: 2
+  System.print("1") // expect: 1
+  System.print("2") // expect: 2
 }.call()
 
 // Extra newlines.
 Fn.new {
 
 
-  IO.print("1") // expect: 1
+  System.print("1") // expect: 1
 
 
-  IO.print("2") // expect: 2
+  System.print("2") // expect: 2
 
 
 }.call()