"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/inside_block/inside_block.wren

@@ -4,7 +4,7 @@ if (true) {
   import "module" for Module
   // expect: ran module
 
-  IO.print(Module) // expect: from module
+  System.print(Module) // expect: from module
 }
 
-IO.print(Module) // expect: outer
+System.print(Module) // expect: outer

test/language/module/inside_block/module.wren

@@ -1,3 +1,3 @@
 // nontest
 var Module = "from module"
-IO.print("ran module")
+System.print("ran module")