"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.

example/hello.wren

@@ -1 +1 @@
-IO.print("Hello, world!")
+System.print("Hello, world!")

example/import-module/lovecraft.wren

@@ -5,4 +5,4 @@ class Lovecraft {
   say() { Cthulu.new().message }
 }
 
-IO.print(Lovecraft.new().say())
+System.print(Lovecraft.new().say())

example/mandelbrot.wren

@@ -29,8 +29,8 @@ for (yPixel in 0...24) {
       }
     }
 
-    IO.write(pixel)
+    System.write(pixel)
   }
 
-  IO.print("")
-}
+  System.print()
+}

example/sokoban.wren

@@ -1 +0,0 @@
-IO.print("Hello, world!")

example/syntax.wren

@@ -9,7 +9,7 @@ class SyntaxExample {
   // Constructor
   construct new() {
     // Top-level name `IO`
-    IO.print("I am a constructor!")
+    System.print("I am a constructor!")
 
     // Method calls
     variables
@@ -58,7 +58,7 @@ class SyntaxExample {
   field=(value) { _field = value }
 
   // Method with arguments
-  print(a, b) { IO.print(a + b) }
+  print(a, b) { System.print(a + b) }
 
   // Operators
   +(other) { "infix + " + other }
@@ -97,7 +97,7 @@ class ReservedWords is SyntaxExample {
 
     // `for`, `in`
     for (beatle in ["george", "john", "paul", "ringo"]) {
-      IO.print(beatle)
+      System.print(beatle)
       // `break`
       break
     }
@@ -147,9 +147,9 @@ class Literals is SyntaxExample {
     "\t" // Tab.
     "\v" // Vertical tab.
     // Unicode code points
-    IO.print("\u0041fgdg\u0b83\u00DE") // "AஃÞ"
+    System.print("\u0041fgdg\u0b83\u00DE") // "AஃÞ"
     // Unencoded bytes
-    IO.print("\x48\x69\x2e") // "Hi."
+    System.print("\x48\x69\x2e") // "Hi."
   }
 
   ranges {