"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.

doc/site/error-handling.markdown

@@ -108,7 +108,7 @@ For example, if you run this program:
     }
 
     var error = fiber.try()
-    IO.print("Caught error: ", error)
+    System.print("Caught error: " + error)
 
 It prints:
 
@@ -120,7 +120,7 @@ usual. When a fiber has been aborted because of a runtime error, you can also
 get the error from the fiber object. Continuing the above example:
 
     :::dart
-    IO.print(fiber.error)
+    System.print(fiber.error)
 
 This also prints: