"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/number/bitwise_rsh.wren

@@ -1,15 +1,15 @@
-IO.print(0 >> 0) // expect: 0
-IO.print(1 >> 0) // expect: 1
-IO.print(0 >> 1) // expect: 0
-IO.print(1 >> 1) // expect: 0
-IO.print(2863311530 >> 1) // expect: 1431655765
-IO.print(4042322160 >> 1) // expect: 2021161080
+System.print(0 >> 0) // expect: 0
+System.print(1 >> 0) // expect: 1
+System.print(0 >> 1) // expect: 0
+System.print(1 >> 1) // expect: 0
+System.print(2863311530 >> 1) // expect: 1431655765
+System.print(4042322160 >> 1) // expect: 2021161080
 
 // Max u32 value.
-IO.print(4294967295 >> 1) // expect: 2147483647
+System.print(4294967295 >> 1) // expect: 2147483647
 
 // Past max u32 value.
-IO.print(4294967296 >> 1) // expect: 0
+System.print(4294967296 >> 1) // expect: 0
 
 // TODO: Negative numbers.
 // TODO: Floating-point numbers.