"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/map/to_string.wren

@@ -1,11 +1,11 @@
 // Handle empty map.
-IO.print({}.toString)             // expect: {}
+System.print({}.toString)             // expect: {}
 
 // Does not quote strings.
-IO.print({"1": "2"}.toString) // expect: {1: 2}
+System.print({"1": "2"}.toString) // expect: {1: 2}
 
 // Nested maps.
-IO.print({1: {2: {}}}) // expect: {1: {2: {}}}
+System.print({1: {2: {}}}) // expect: {1: {2: {}}}
 
 // Calls toString on elements.
 class Foo {
@@ -13,12 +13,12 @@ class Foo {
   toString { "Foo.toString" }
 }
 
-IO.print({1: Foo.new()}) // expect: {1: Foo.toString}
+System.print({1: Foo.new()}) // expect: {1: Foo.toString}
 
 // Since iteration order is unspecified, we don't know what order the results
 // will be.
 var s = {1: 2, 3: 4, 5: 6}.toString
-IO.print(s == "{1: 2, 3: 4, 5: 6}" ||
+System.print(s == "{1: 2, 3: 4, 5: 6}" ||
          s == "{1: 2, 5: 6, 3: 4}" ||
          s == "{3: 4, 1: 2, 5: 6}" ||
          s == "{3: 4, 5: 6, 1: 2}" ||