"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/variable/global_without_initializer.wren

@@ -1,2 +1,2 @@
 var a
-IO.print(a) // expect: null
+System.print(a) // expect: null

test/language/variable/local_in_initializer.wren

@@ -1,4 +1,4 @@
 {
   var a = a + 1 // expect error
-  IO.print(a)
+  System.print(a)
 }

test/language/variable/local_in_middle_of_block.wren

@@ -3,13 +3,13 @@ class Foo {
 
   bar {
     var a = "a"
-    IO.print(a) // expect: a
+    System.print(a) // expect: a
     var b = a + " b"
-    IO.print(b) // expect: a b
+    System.print(b) // expect: a b
     var c = a + " c"
-    IO.print(c) // expect: a c
+    System.print(c) // expect: a c
     var d = b + " d"
-    IO.print(d) // expect: a b d
+    System.print(d) // expect: a b d
   }
 }
 

test/language/variable/local_in_nested_block.wren

@@ -1,6 +1,6 @@
 {
   var a = "outer"
   {
-    IO.print(a) // expect: outer
+    System.print(a) // expect: outer
   }
 }

test/language/variable/local_without_initializer.wren

@@ -1,4 +1,4 @@
 {
   var a
-  IO.print(a) // expect: null
+  System.print(a) // expect: null
 }

test/language/variable/many_locals.wren

@@ -255,5 +255,5 @@
   var a253 = a252
   var a254 = a253
   var a255 = a254
-  IO.print(a255) // expect: value
+  System.print(a255) // expect: value
 }

test/language/variable/many_nonsimultaneous_locals.wren

@@ -259,7 +259,7 @@
     var a253 = a252
     var a254 = a253
     var a255 = a254
-    IO.print(a255) // expect: value a
+    System.print(a255) // expect: value a
   }
 
   {
@@ -519,6 +519,6 @@
     var b253 = b252
     var b254 = b253
     var b255 = b254
-    IO.print(b255) // expect: value b
+    System.print(b255) // expect: value b
   }
 }

test/language/variable/outside_method.wren

@@ -6,13 +6,13 @@ class Foo {
   foo { "method" }
 
   method {
-    IO.print(foo)
+    System.print(foo)
   }
 
   static foo { "class method" }
 
   static classMethod {
-    IO.print(foo)
+    System.print(foo)
   }
 }
 

test/language/variable/scope_if.wren

@@ -3,11 +3,11 @@ var a = "out"
 if (true) {
   var a = "in"
 }
-IO.print(a) // expect: out
+System.print(a) // expect: out
 
 // Create a local scope for the 'else' expression.
 var b = "out"
 if (false) "dummy" else {
   var b = "in"
 }
-IO.print(b) // expect: out
+System.print(b) // expect: out

test/language/variable/scope_reuse_in_different_blocks.wren

@@ -1,9 +1,9 @@
 {
   var a = "first"
-  IO.print(a) // expect: first
+  System.print(a) // expect: first
 }
 
 {
   var a = "second"
-  IO.print(a) // expect: second
+  System.print(a) // expect: second
 }

test/language/variable/scope_while.wren

@@ -4,4 +4,4 @@ var i = 0
 while ((i = i + 1) <= 1) {
   var a = "inner"
 }
-IO.print(a) // expect: outer
+System.print(a) // expect: outer

test/language/variable/shadow_and_local.wren

@@ -1,8 +1,8 @@
 {
   var a = "outer"
   {
-    IO.print(a) // expect: outer
+    System.print(a) // expect: outer
     var a = "inner"
-    IO.print(a) // expect: inner
+    System.print(a) // expect: inner
   }
 }

test/language/variable/shadow_global.wren

@@ -1,6 +1,6 @@
 var a = "global"
 {
   var a = "shadow"
-  IO.print(a) // expect: shadow
+  System.print(a) // expect: shadow
 }
-IO.print(a) // expect: global
+System.print(a) // expect: global

test/language/variable/shadow_in_initializer.wren

@@ -2,6 +2,6 @@
   var a = "outer"
   {
     var a = a + " inner"
-    IO.print(a) // expect: outer inner
+    System.print(a) // expect: outer inner
   }
 }

test/language/variable/shadow_local.wren

@@ -2,7 +2,7 @@
   var a = "local"
   {
     var a = "shadow"
-    IO.print(a) // expect: shadow
+    System.print(a) // expect: shadow
   }
-  IO.print(a) // expect: local
+  System.print(a) // expect: local
 }

test/language/variable/undefined_global.wren

@@ -1 +1 @@
-IO.print(notDefined)  // expect error
+System.print(notDefined)  // expect error

test/language/variable/undefined_local.wren

@@ -1,3 +1,3 @@
 Fn.new {
-  IO.print(notDefined)  // expect error
+  System.print(notDefined)  // expect error
 }.call()