"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/README.md

@@ -8,9 +8,6 @@ This contains the automated validation suite for the VM and built-in libraries.
   classes. If a bug is in `wren_core.c` or `wren_value.c`, it will most likely
   break one of these tests.
 
-* `io/` - Tests for the built in IO library. In other words, methods on the
-  `IO` class. If a bug is in `wren_io.c`, it should break one of these tests.
-
 * `language/` - Tests of the language itself, its grammar and runtime
   semantics. If a bug is in `wren_compiler.c` or `wren_vm.c`, it will most
   likely break one of these tests. This includes tests for the syntax for the