"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/closure/assign_to_closure.wren

@@ -4,15 +4,15 @@ var g = null
 {
   var local = "local"
   f = Fn.new {
-    IO.print(local)
+    System.print(local)
     local = "after f"
-    IO.print(local)
+    System.print(local)
   }
 
   g = Fn.new {
-    IO.print(local)
+    System.print(local)
     local = "after g"
-    IO.print(local)
+    System.print(local)
   }
 }
 

test/language/closure/close_over_function_parameter.wren

@@ -2,7 +2,7 @@ var f = null
 
 Fn.new {|param|
   f = Fn.new {
-    IO.print(param)
+    System.print(param)
   }
 }.call("param")
 

test/language/closure/close_over_later_variable.wren

@@ -7,7 +7,7 @@ Fn.new {
   var a = "a"
   var b = "b"
   Fn.new {
-    IO.print(b) // expect: b
-    IO.print(a) // expect: a
+    System.print(b) // expect: b
+    System.print(a) // expect: a
   }.call()
 }.call()

test/language/closure/close_over_method_parameter.wren

@@ -5,7 +5,7 @@ class Foo {
 
   method(param) {
     F = Fn.new {
-      IO.print(param)
+      System.print(param)
     }
   }
 }

test/language/closure/closed_closure_in_function.wren

@@ -3,7 +3,7 @@ var f = null
 {
   var local = "local"
   f = Fn.new {
-    IO.print(local)
+    System.print(local)
   }
 }
 

test/language/closure/closed_closure_in_method.wren

@@ -7,7 +7,7 @@ var foo = null
     construct new() {}
 
     method {
-      IO.print(local)
+      System.print(local)
     }
   }
 

test/language/closure/nested_closure.wren

@@ -7,9 +7,9 @@ Fn.new {
     Fn.new {
       var c = "c"
       f = Fn.new {
-        IO.print(a)
-        IO.print(b)
-        IO.print(c)
+        System.print(a)
+        System.print(b)
+        System.print(c)
       }
     }.call()
   }.call()

test/language/closure/open_closure_in_function.wren

@@ -1,6 +1,6 @@
 {
   var local = "local"
   Fn.new {
-    IO.print(local) // expect: local
+    System.print(local) // expect: local
   }.call()
 }

test/language/closure/open_closure_in_method.wren

@@ -5,7 +5,7 @@
     construct new() {}
 
     method {
-      IO.print(local)
+      System.print(local)
     }
   }
 

test/language/closure/reference_closure_multiple_times.wren

@@ -3,8 +3,8 @@ var f = null
 {
   var a = "a"
   f = Fn.new {
-    IO.print(a)
-    IO.print(a)
+    System.print(a)
+    System.print(a)
   }
 }
 

test/language/closure/reuse_closure_slot.wren

@@ -3,7 +3,7 @@
 
   {
     var a = "a"
-    f = Fn.new { IO.print(a) }
+    f = Fn.new { System.print(a) }
   }
 
   {

test/language/closure/shadow_closure_with_local.wren

@@ -2,10 +2,10 @@
   var foo = "closure"
   Fn.new {
     {
-      IO.print(foo) // expect: closure
+      System.print(foo) // expect: closure
       var foo = "shadow"
-      IO.print(foo) // expect: shadow
+      System.print(foo) // expect: shadow
     }
-    IO.print(foo) // expect: closure
+    System.print(foo) // expect: closure
   }.call()
 }