"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_not.wren

@@ -1,22 +1,22 @@
-IO.print(~0) // expect: 4294967295
-IO.print(~1) // expect: 4294967294
-IO.print(~23) // expect: 4294967272
+System.print(~0) // expect: 4294967295
+System.print(~1) // expect: 4294967294
+System.print(~23) // expect: 4294967272
 
 // Max u32 value.
-IO.print(~4294967295) // expect: 0
+System.print(~4294967295) // expect: 0
 
 // Past max u32 value.
-IO.print(~4294967296) // expect: 4294967295
+System.print(~4294967296) // expect: 4294967295
 
 // Negative numbers.
-IO.print(~-1) // expect: 0
-IO.print(~-123) // expect: 122
-IO.print(~-4294967294) // expect: 4294967293
-IO.print(~-4294967295) // expect: 4294967294
-IO.print(~-4294967296) // expect: 4294967295
+System.print(~-1) // expect: 0
+System.print(~-123) // expect: 122
+System.print(~-4294967294) // expect: 4294967293
+System.print(~-4294967295) // expect: 4294967294
+System.print(~-4294967296) // expect: 4294967295
 
 // Floating point values.
-IO.print(~1.23) // expect: 4294967294
-IO.print(~0.00123) // expect: 4294967295
-IO.print(~345.67) // expect: 4294966950
-IO.print(~-12.34) // expect: 11
+System.print(~1.23) // expect: 4294967294
+System.print(~0.00123) // expect: 4294967295
+System.print(~345.67) // expect: 4294966950
+System.print(~-12.34) // expect: 11