"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/number/hex_literals.wren

@@ -1,8 +1,8 @@
 var x = 0xFF
 
-IO.print(x) // expect: 255
-IO.print(x + 1) // expect: 256
-IO.print(x == 255) // expect: true
-IO.print(0x09 is Num) // expect: true
-IO.print(x is Num) // expect: true
-IO.print(-0xFF) // expect: -255
+System.print(x) // expect: 255
+System.print(x + 1) // expect: 256
+System.print(x == 255) // expect: true
+System.print(0x09 is Num) // expect: true
+System.print(x is Num) // expect: true
+System.print(-0xFF) // expect: -255

test/language/number/literals.wren

@@ -1,9 +1,9 @@
-IO.print(123)     // expect: 123
-IO.print(987654)  // expect: 987654
-IO.print(0)       // expect: 0
-IO.print(-0)      // expect: -0
+System.print(123)     // expect: 123
+System.print(987654)  // expect: 987654
+System.print(0)       // expect: 0
+System.print(-0)      // expect: -0
 
-IO.print(123.456) // expect: 123.456
-IO.print(-0.001)  // expect: -0.001
+System.print(123.456) // expect: 123.456
+System.print(-0.001)  // expect: -0.001
 
 // TODO: Literals at and beyond numeric limits.

test/language/number/scientific_literals.wren

@@ -1,9 +1,9 @@
 var x = 2.55e2
 
-IO.print(x) // expect: 255
-IO.print(x + 1) // expect: 256
-IO.print(x == 255) // expect: true
-IO.print(2.55e-2 is Num) // expect: true
-IO.print(x is Num) // expect: true
-IO.print(-2.55e2) // expect: -255
-IO.print(-25500e-2) // expect: -255
+System.print(x) // expect: 255
+System.print(x + 1) // expect: 256
+System.print(x == 255) // expect: true
+System.print(2.55e-2 is Num) // expect: true
+System.print(x is Num) // expect: true
+System.print(-2.55e2) // expect: -255
+System.print(-25500e-2) // expect: -255