"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/function/arity.wren

@@ -1,5 +1,5 @@
-IO.print(Fn.new {}.arity) // expect: 0
-IO.print(Fn.new {|a| a}.arity) // expect: 1
-IO.print(Fn.new {|a, b| a}.arity) // expect: 2
-IO.print(Fn.new {|a, b, c| a}.arity) // expect: 3
-IO.print(Fn.new {|a, b, c, d| a}.arity) // expect: 4
+System.print(Fn.new {}.arity) // expect: 0
+System.print(Fn.new {|a| a}.arity) // expect: 1
+System.print(Fn.new {|a, b| a}.arity) // expect: 2
+System.print(Fn.new {|a, b, c| a}.arity) // expect: 3
+System.print(Fn.new {|a, b, c, d| a}.arity) // expect: 4

test/core/function/call_extra_arguments.wren

@@ -1,7 +1,7 @@
-var f0 = Fn.new { IO.print("zero") }
-var f1 = Fn.new {|a| IO.print("one ", a) }
-var f2 = Fn.new {|a, b| IO.print("two ", a, " ", b) }
-var f3 = Fn.new {|a, b, c| IO.print("three ", a, " ", b, " ", c) }
+var f0 = Fn.new { System.print("zero") }
+var f1 = Fn.new {|a| System.print("one " + a) }
+var f2 = Fn.new {|a, b| System.print("two " + a + " " + b) }
+var f3 = Fn.new {|a, b, c| System.print("three " + a + " " + b + " " + c) }
 
 f0.call("a") // expect: zero
 f0.call("a", "b") // expect: zero

test/core/function/call_missing_arguments.wren

@@ -1,2 +1,2 @@
-var f2 = Fn.new {|a, b| IO.print(a, b) }
+var f2 = Fn.new {|a, b| System.print(a, b) }
 f2.call("a") // expect runtime error: Function expects more arguments.

test/core/function/equality.wren

@@ -1,16 +1,16 @@
 // Not structurally equal.
-IO.print(Fn.new { 123 } == Fn.new { 123 })  // expect: false
-IO.print(Fn.new { 123 } != Fn.new { 123 })  // expect: true
+System.print(Fn.new { 123 } == Fn.new { 123 })  // expect: false
+System.print(Fn.new { 123 } != Fn.new { 123 })  // expect: true
 
 // Not equal to other types.
-IO.print(Fn.new { 123 } == 1)         // expect: false
-IO.print(Fn.new { 123 } == false)     // expect: false
-IO.print(Fn.new { 123 } == "fn 123")  // expect: false
-IO.print(Fn.new { 123 } != 1)         // expect: true
-IO.print(Fn.new { 123 } != false)     // expect: true
-IO.print(Fn.new { 123 } != "fn 123")  // expect: true
+System.print(Fn.new { 123 } == 1)         // expect: false
+System.print(Fn.new { 123 } == false)     // expect: false
+System.print(Fn.new { 123 } == "fn 123")  // expect: false
+System.print(Fn.new { 123 } != 1)         // expect: true
+System.print(Fn.new { 123 } != false)     // expect: true
+System.print(Fn.new { 123 } != "fn 123")  // expect: true
 
 // Equal by identity.
 var f = Fn.new { 123 }
-IO.print(f == f) // expect: true
-IO.print(f != f) // expect: false
+System.print(f == f) // expect: true
+System.print(f != f) // expect: false

test/core/function/to_string.wren

@@ -1 +1 @@
-IO.print(Fn.new {}) // expect: <fn>
+System.print(Fn.new {}) // expect: <fn>

test/core/function/type.wren

@@ -1,4 +1,4 @@
-IO.print(Fn.new { 0 } is Fn)      // expect: true
-IO.print(Fn.new { 0 } is Object)  // expect: true
-IO.print(Fn.new { 0 } is String)  // expect: false
-IO.print(Fn.new { 0 }.type == Fn) // expect: true
+System.print(Fn.new { 0 } is Fn)      // expect: true
+System.print(Fn.new { 0 } is Object)  // expect: true
+System.print(Fn.new { 0 } is String)  // expect: false
+System.print(Fn.new { 0 }.type == Fn) // expect: true