"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/sequence/where.wren

@@ -29,14 +29,14 @@ class Fib is Sequence {
 var largeFibs = Fib.new().where {|fib| fib > 100 }
 var iterator = null
 
-IO.print(largeFibs is Sequence) // expect: true
-IO.print(largeFibs) // expect: instance of WhereSequence
+System.print(largeFibs is Sequence) // expect: true
+System.print(largeFibs) // expect: instance of WhereSequence
 
 iterator = largeFibs.iterate(iterator)
-IO.print(largeFibs.iteratorValue(iterator)) // expect: 144
+System.print(largeFibs.iteratorValue(iterator)) // expect: 144
 
 iterator = largeFibs.iterate(iterator)
-IO.print(largeFibs.iteratorValue(iterator)) // expect: 233
+System.print(largeFibs.iteratorValue(iterator)) // expect: 233
 
 iterator = largeFibs.iterate(iterator)
-IO.print(largeFibs.iteratorValue(iterator)) // expect: 377
+System.print(largeFibs.iteratorValue(iterator)) // expect: 377