"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/is/is.wren

@@ -1,33 +1,33 @@
-IO.print(Num is Class) // expect: true
-IO.print(true is Bool) // expect: true
-IO.print(Fn.new { 1 } is Fn) // expect: true
-IO.print(123 is Num) // expect: true
-IO.print(null is Null) // expect: true
-IO.print("s" is String) // expect: true
+System.print(Num is Class) // expect: true
+System.print(true is Bool) // expect: true
+System.print(Fn.new { 1 } is Fn) // expect: true
+System.print(123 is Num) // expect: true
+System.print(null is Null) // expect: true
+System.print("s" is String) // expect: true
 
-IO.print(Num is Bool) // expect: false
-IO.print(null is Class) // expect: false
-IO.print(true is Fn) // expect: false
-IO.print(Fn.new { 1 } is Num) // expect: false
-IO.print("s" is Null) // expect: false
-IO.print(123 is String) // expect: false
+System.print(Num is Bool) // expect: false
+System.print(null is Class) // expect: false
+System.print(true is Fn) // expect: false
+System.print(Fn.new { 1 } is Num) // expect: false
+System.print("s" is Null) // expect: false
+System.print(123 is String) // expect: false
 
 // Everything extends Object.
-IO.print(Num is Object) // expect: true
-IO.print(null is Object) // expect: true
-IO.print(true is Object) // expect: true
-IO.print(Fn.new { 1 } is Object) // expect: true
-IO.print("s" is Object) // expect: true
-IO.print(123 is Object) // expect: true
+System.print(Num is Object) // expect: true
+System.print(null is Object) // expect: true
+System.print(true is Object) // expect: true
+System.print(Fn.new { 1 } is Object) // expect: true
+System.print("s" is Object) // expect: true
+System.print(123 is Object) // expect: true
 
 // Classes extend Class.
-IO.print(Num is Class) // expect: true
-IO.print(null is Class) // expect: false
-IO.print(true is Class) // expect: false
-IO.print(Fn.new { 1 } is Class) // expect: false
-IO.print("s" is Class) // expect: false
-IO.print(123 is Class) // expect: false
+System.print(Num is Class) // expect: true
+System.print(null is Class) // expect: false
+System.print(true is Class) // expect: false
+System.print(Fn.new { 1 } is Class) // expect: false
+System.print("s" is Class) // expect: false
+System.print(123 is Class) // expect: false
 
 // Ignore newline after "is".
-IO.print(123 is
+System.print(123 is
   Num) // expect: true