"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/equality.wren

@@ -1,19 +1,19 @@
-IO.print(123 == 123)  // expect: true
-IO.print(123 == 124)  // expect: false
-IO.print(-3 == 3)     // expect: false
-IO.print(0 == -0)     // expect: true
+System.print(123 == 123)  // expect: true
+System.print(123 == 124)  // expect: false
+System.print(-3 == 3)     // expect: false
+System.print(0 == -0)     // expect: true
 
 // Not equal to other types.
-IO.print(123 == "123") // expect: false
-IO.print(1 == true)    // expect: false
-IO.print(0 == false)   // expect: false
+System.print(123 == "123") // expect: false
+System.print(1 == true)    // expect: false
+System.print(0 == false)   // expect: false
 
-IO.print(123 != 123)  // expect: false
-IO.print(123 != 124)  // expect: true
-IO.print(-3 != 3)     // expect: true
-IO.print(0 != -0)     // expect: false
+System.print(123 != 123)  // expect: false
+System.print(123 != 124)  // expect: true
+System.print(-3 != 3)     // expect: true
+System.print(0 != -0)     // expect: false
 
 // Not equal to other types.
-IO.print(123 != "123") // expect: true
-IO.print(1 != true)    // expect: true
-IO.print(0 != false)   // expect: true
+System.print(123 != "123") // expect: true
+System.print(1 != true)    // expect: true
+System.print(0 != false)   // expect: true