"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/comparison.wren

@@ -1,27 +1,27 @@
-IO.print(1 < 2)    // expect: true
-IO.print(2 < 2)    // expect: false
-IO.print(2 < 1)    // expect: false
+System.print(1 < 2)    // expect: true
+System.print(2 < 2)    // expect: false
+System.print(2 < 1)    // expect: false
 
-IO.print(1 <= 2)    // expect: true
-IO.print(2 <= 2)    // expect: true
-IO.print(2 <= 1)    // expect: false
+System.print(1 <= 2)    // expect: true
+System.print(2 <= 2)    // expect: true
+System.print(2 <= 1)    // expect: false
 
-IO.print(1 > 2)    // expect: false
-IO.print(2 > 2)    // expect: false
-IO.print(2 > 1)    // expect: true
+System.print(1 > 2)    // expect: false
+System.print(2 > 2)    // expect: false
+System.print(2 > 1)    // expect: true
 
-IO.print(1 >= 2)    // expect: false
-IO.print(2 >= 2)    // expect: true
-IO.print(2 >= 1)    // expect: true
+System.print(1 >= 2)    // expect: false
+System.print(2 >= 2)    // expect: true
+System.print(2 >= 1)    // expect: true
 
 // Zero and negative zero compare the same.
-IO.print(0 < -0) // expect: false
-IO.print(-0 < 0) // expect: false
-IO.print(0 > -0) // expect: false
-IO.print(-0 > 0) // expect: false
-IO.print(0 <= -0) // expect: true
-IO.print(-0 <= 0) // expect: true
-IO.print(0 >= -0) // expect: true
-IO.print(-0 >= 0) // expect: true
+System.print(0 < -0) // expect: false
+System.print(-0 < 0) // expect: false
+System.print(0 > -0) // expect: false
+System.print(-0 > 0) // expect: false
+System.print(0 <= -0) // expect: true
+System.print(-0 <= 0) // expect: true
+System.print(0 >= -0) // expect: true
+System.print(-0 >= 0) // expect: true
 
 // TODO: Wrong type for RHS.