"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/truncate.wren

@@ -1,13 +1,13 @@
-IO.print(123.truncate)      // expect: 123
-IO.print((-123).truncate)   // expect: -123
-IO.print(0.truncate)        // expect: 0
-IO.print((-0).truncate)     // expect: -0
-IO.print(0.123.truncate)    // expect: 0
-IO.print(12.3.truncate)     // expect: 12
-IO.print((-0.123).truncate) // expect: -0
-IO.print((-12.3).truncate)  // expect: -12
+System.print(123.truncate)      // expect: 123
+System.print((-123).truncate)   // expect: -123
+System.print(0.truncate)        // expect: 0
+System.print((-0).truncate)     // expect: -0
+System.print(0.123.truncate)    // expect: 0
+System.print(12.3.truncate)     // expect: 12
+System.print((-0.123).truncate) // expect: -0
+System.print((-12.3).truncate)  // expect: -12
 
 // Using 32-bit representation, values "beyond" those  two will lead to
 // approximation.
-IO.print((12345678901234.5).truncate)   // expect: 12345678901234
-IO.print((-12345678901234.5).truncate)  // expect: -12345678901234
+System.print((12345678901234.5).truncate)   // expect: 12345678901234
+System.print((-12345678901234.5).truncate)  // expect: -12345678901234