"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/api/foreign_class.wren

@@ -11,27 +11,27 @@ foreign class Counter {
 }
 
 var counter = Counter.new()
-IO.print(counter.value) // expect: 0
+System.print(counter.value) // expect: 0
 counter.increment(3.1)
-IO.print(counter.value) // expect: 3.1
+System.print(counter.value) // expect: 3.1
 counter.increment(1.2)
-IO.print(counter.value) // expect: 4.3
+System.print(counter.value) // expect: 4.3
 
 // Foreign classes can inherit a class as long as it has no fields.
 class PointBase {
   inherited() {
-    IO.print("inherited method")
+    System.print("inherited method")
   }
 }
 
 // Class with non-default constructor.
 foreign class Point is PointBase {
   construct new() {
-    IO.print("default")
+    System.print("default")
   }
 
   construct new(x, y, z) {
-    IO.print(x, ", ", y, ", ", z)
+    System.print(x.toString + ", " + y.toString + ", " + z.toString)
   }
 
   foreign translate(x, y, z)
@@ -39,19 +39,19 @@ foreign class Point is PointBase {
 }
 
 var p = Point.new(1, 2, 3) // expect: 1, 2, 3
-IO.print(p) // expect: (1, 2, 3)
+System.print(p) // expect: (1, 2, 3)
 p.translate(3, 4, 5)
-IO.print(p) // expect: (4, 6, 8)
+System.print(p) // expect: (4, 6, 8)
 
 p = Point.new() // expect: default
-IO.print(p) // expect: (0, 0, 0)
+System.print(p) // expect: (0, 0, 0)
 
 p.inherited() // expect: inherited method
 
 var error = Fiber.new {
   class Subclass is Point {}
 }.try()
-IO.print(error) // expect: Class 'Subclass' cannot inherit from foreign class 'Point'.
+System.print(error) // expect: Class 'Subclass' cannot inherit from foreign class 'Point'.
 
 // Class with a finalizer.
 foreign class Resource {
@@ -65,14 +65,14 @@ var resources = [
 ]
 
 Api.gc()
-IO.print(Api.finalized) // expect: 0
+System.print(Api.finalized) // expect: 0
 
 resources.removeAt(-1)
 
 Api.gc()
-IO.print(Api.finalized) // expect: 1
+System.print(Api.finalized) // expect: 1
 
 resources.clear()
 
 Api.gc()
-IO.print(Api.finalized) // expect: 3
+System.print(Api.finalized) // expect: 3

test/api/returns.wren

@@ -8,10 +8,10 @@ class Api {
   foreign static returnFalse
 }
 
-IO.print(Api.implicitNull == null) // expect: true
+System.print(Api.implicitNull == null) // expect: true
 
-IO.print(Api.returnInt)  // expect: 123456
-IO.print(Api.returnFloat) // expect: 123.456
+System.print(Api.returnInt)  // expect: 123456
+System.print(Api.returnFloat) // expect: 123.456
 
-IO.print(Api.returnTrue)  // expect: true
-IO.print(Api.returnFalse) // expect: false
+System.print(Api.returnTrue)  // expect: true
+System.print(Api.returnFalse) // expect: false

test/api/value.wren

@@ -11,4 +11,4 @@ for (i in 1...10) {
   s = s + " more"
 }
 
-IO.print(Api.value) // expect: [list, of, strings]
+System.print(Api.value) // expect: [list, of, strings]