"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/module/multiple_variables/module.wren

@@ -5,4 +5,4 @@ var Module3 = "from module three"
 var Module4 = "from module four"
 var Module5 = "from module five"
 
-IO.print("ran module")
+System.print("ran module")

test/language/module/multiple_variables/multiple_variables.wren

@@ -3,8 +3,8 @@ import "module" for Module1, Module2, Module3, Module4, Module5
 // Only execute module body once:
 // expect: ran module
 
-IO.print(Module1) // expect: from module one
-IO.print(Module2) // expect: from module two
-IO.print(Module3) // expect: from module three
-IO.print(Module4) // expect: from module four
-IO.print(Module5) // expect: from module five
+System.print(Module1) // expect: from module one
+System.print(Module2) // expect: from module two
+System.print(Module3) // expect: from module three
+System.print(Module4) // expect: from module four
+System.print(Module5) // expect: from module five