"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/implicit_receiver/inherited_methods.wren

@@ -2,15 +2,15 @@ class Foo {
   construct new() {}
 
   getter {
-    IO.print("getter")
+    System.print("getter")
   }
 
   setter=(value) {
-    IO.print("setter")
+    System.print("setter")
   }
 
   method(a) {
-    IO.print("method")
+    System.print("method")
   }
 }
 

test/language/implicit_receiver/instance_methods.wren

@@ -2,15 +2,15 @@ class Foo {
   construct new() {}
 
   getter {
-    IO.print("getter")
+    System.print("getter")
   }
 
   setter=(value) {
-    IO.print("setter")
+    System.print("setter")
   }
 
   method(a) {
-    IO.print("method")
+    System.print("method")
   }
 
   test {

test/language/implicit_receiver/locals_shadow_getter.wren

@@ -4,15 +4,15 @@ class Foo {
   bar { "getter" }
 
   test {
-    IO.print(bar) // expect: getter
+    System.print(bar) // expect: getter
 
     {
-      IO.print(bar) // expect: getter
+      System.print(bar) // expect: getter
       var bar = "local"
-      IO.print(bar) // expect: local
+      System.print(bar) // expect: local
     }
 
-    IO.print(bar) // expect: getter
+    System.print(bar) // expect: getter
   }
 }
 

test/language/implicit_receiver/locals_shadow_setter.wren

@@ -2,7 +2,7 @@ class Foo {
   construct new() {}
 
   bar=(value) {
-    IO.print("setter")
+    System.print("setter")
     return value
   }
 

test/language/implicit_receiver/nested_class.wren

@@ -2,15 +2,15 @@ class Outer {
   construct new() {}
 
   getter {
-    IO.print("outer getter")
+    System.print("outer getter")
   }
 
   setter=(value) {
-    IO.print("outer setter")
+    System.print("outer setter")
   }
 
   method(a) {
-    IO.print("outer method")
+    System.print("outer method")
   }
 
   test {
@@ -22,15 +22,15 @@ class Outer {
       construct new() {}
 
       getter {
-        IO.print("inner getter")
+        System.print("inner getter")
       }
 
       setter=(value) {
-        IO.print("inner setter")
+        System.print("inner setter")
       }
 
       method(a) {
-        IO.print("inner method")
+        System.print("inner method")
       }
 
       test {

test/language/implicit_receiver/static_methods.wren

@@ -1,14 +1,14 @@
 class Foo {
   static getter {
-    IO.print("getter")
+    System.print("getter")
   }
 
   static setter=(value) {
-    IO.print("setter")
+    System.print("setter")
   }
 
   static method(a) {
-    IO.print("method")
+    System.print("method")
   }
 
   static test {