"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/while/closure_in_body.wren

@@ -3,7 +3,7 @@ var list = []
 var i = 1
 while (i < 4) {
   var j = i + 1
-  list.add(Fn.new { IO.print(j) })
+  list.add(Fn.new { System.print(j) })
   i = i + 1
 }
 

test/language/while/newline_after_while.wren

@@ -1,2 +1,2 @@
 while // expect error
-(true) IO.print("bad")
+(true) System.print("bad")

test/language/while/return_closure.wren

@@ -1,7 +1,7 @@
 var f = Fn.new {
   while (true) {
     var i = "i"
-    return Fn.new { IO.print(i) }
+    return Fn.new { System.print(i) }
   }
 }
 

test/language/while/return_inside.wren

@@ -5,5 +5,5 @@ var f = Fn.new {
   }
 }
 
-IO.print(f.call())
+System.print(f.call())
 // expect: i

test/language/while/syntax.wren

@@ -1,6 +1,6 @@
 // Single-expression body.
 var c = 0
-while (c < 3) IO.print(c = c + 1)
+while (c < 3) System.print(c = c + 1)
 // expect: 1
 // expect: 2
 // expect: 3
@@ -8,7 +8,7 @@ while (c < 3) IO.print(c = c + 1)
 // Block body.
 var a = 0
 while (a < 3) {
-  IO.print(a)
+  System.print(a)
   a = a + 1
 }
 // expect: 0

test/language/while/truth.wren

@@ -1,26 +1,26 @@
 // False and null are false.
 while (false) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 while (null) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 // Everything else is true.
 while (true) {
-  IO.print("true") // expect: true
+  System.print("true") // expect: true
   break
 }
 
 while (0) {
-  IO.print(0) // expect: 0
+  System.print(0) // expect: 0
   break
 }
 
 while ("") {
-  IO.print("string") // expect: string
+  System.print("string") // expect: string
   break
 }