"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/bitwise_precedence.wren

@@ -1,23 +1,23 @@
 // << have higher precedence than |.
-IO.print(2 | 1 << 1) // expect: 2
-IO.print(1 << 1 | 2) // expect: 2
+System.print(2 | 1 << 1) // expect: 2
+System.print(1 << 1 | 2) // expect: 2
 
 // << has higher precedence than &.
-IO.print(2 & 1 << 1) // expect: 2
-IO.print(1 << 1 & 2) // expect: 2
+System.print(2 & 1 << 1) // expect: 2
+System.print(1 << 1 & 2) // expect: 2
 
 // << has higher precedence than ^.
-IO.print(2 ^ 1 << 1) // expect: 0
-IO.print(1 << 1 ^ 2) // expect: 0
+System.print(2 ^ 1 << 1) // expect: 0
+System.print(1 << 1 ^ 2) // expect: 0
 
 // & has higher precedence than |.
-IO.print(1 & 1 | 2) // expect: 3
-IO.print(2 | 1 & 1) // expect: 3
+System.print(1 & 1 | 2) // expect: 3
+System.print(2 | 1 & 1) // expect: 3
 
 // & has higher precedence than ^.
-IO.print(1 & 1 ^ 2) // expect: 3
-IO.print(2 ^ 1 & 1) // expect: 3
+System.print(1 & 1 ^ 2) // expect: 3
+System.print(2 ^ 1 & 1) // expect: 3
 
 // ^ has higher precedence than |.
-IO.print(1 ^ 1 | 1) // expect: 1
-IO.print(1 | 1 ^ 1) // expect: 1
+System.print(1 ^ 1 | 1) // expect: 1
+System.print(1 | 1 ^ 1) // expect: 1