"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.

README.md

@@ -4,11 +4,11 @@ Think Smalltalk in a Lua-sized package with a dash of Erlang and wrapped up in
 a familiar, modern [syntax][].
 
 ```dart
-IO.print("Hello, world!")
+System.print("Hello, world!")
 
 class Wren {
   flyTo(city) {
-    IO.print("Flying to ", city)
+    System.print("Flying to ", city)
   }
 }
 
@@ -16,7 +16,7 @@ var adjectives = Fiber.new {
   ["small", "clean", "fast"].each {|word| Fiber.yield(word) }
 }
 
-while (!adjectives.isDone) IO.print(adjectives.call())
+while (!adjectives.isDone) System.print(adjectives.call())
 ```
 
  *  **Wren is small.** The VM implementation is under [4,000 semicolons][src].