"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/bool/equality.wren

@@ -1,23 +1,23 @@
-IO.print(true == true)    // expect: true
-IO.print(true == false)   // expect: false
-IO.print(false == true)   // expect: false
-IO.print(false == false)  // expect: true
+System.print(true == true)    // expect: true
+System.print(true == false)   // expect: false
+System.print(false == true)   // expect: false
+System.print(false == false)  // expect: true
 
 // Not equal to other types.
-IO.print(true == 1)        // expect: false
-IO.print(false == 0)       // expect: false
-IO.print(true == "true")   // expect: false
-IO.print(false == "false") // expect: false
-IO.print(false == "")      // expect: false
+System.print(true == 1)        // expect: false
+System.print(false == 0)       // expect: false
+System.print(true == "true")   // expect: false
+System.print(false == "false") // expect: false
+System.print(false == "")      // expect: false
 
-IO.print(true != true)    // expect: false
-IO.print(true != false)   // expect: true
-IO.print(false != true)   // expect: true
-IO.print(false != false)  // expect: false
+System.print(true != true)    // expect: false
+System.print(true != false)   // expect: true
+System.print(false != true)   // expect: true
+System.print(false != false)  // expect: false
 
 // Not equal to other types.
-IO.print(true != 1)        // expect: true
-IO.print(false != 0)       // expect: true
-IO.print(true != "true")   // expect: true
-IO.print(false != "false") // expect: true
-IO.print(false != "")      // expect: true
+System.print(true != 1)        // expect: true
+System.print(false != 0)       // expect: true
+System.print(true != "true")   // expect: true
+System.print(false != "false") // expect: true
+System.print(false != "")      // expect: true

test/core/bool/not.wren

@@ -1,3 +1,3 @@
-IO.print(!true)    // expect: false
-IO.print(!false)   // expect: true
-IO.print(!!true)   // expect: true
+System.print(!true)    // expect: false
+System.print(!false)   // expect: true
+System.print(!!true)   // expect: true

test/core/bool/to_string.wren

@@ -1,2 +1,2 @@
-IO.print(true.toString)  // expect: true
-IO.print(false.toString) // expect: false
+System.print(true.toString)  // expect: true
+System.print(false.toString) // expect: false

test/core/bool/type.wren

@@ -1,4 +1,4 @@
-IO.print(true is Bool)      // expect: true
-IO.print(true is Object)    // expect: true
-IO.print(true is Num)       // expect: false
-IO.print(true.type == Bool) // expect: true
+System.print(true is Bool)      // expect: true
+System.print(true is Object)    // expect: true
+System.print(true is Num)       // expect: false
+System.print(true.type == Bool) // expect: true