"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/getting-started.markdown

@@ -49,12 +49,12 @@ interactive mode. You can type in a line of code, and it will immediately
 execute it. Here's something to try:
 
     :::dart
-    IO.print("Hello, world!")
+    System.print("Hello, world!")
 
 Or a little more exciting:
 
     :::dart
-    for (i in 1..10) IO.print("Counting up ", i)
+    for (i in 1..10) System.print("Counting up " + i.toString)
 
 You can exit the interpreter using good old Ctrl-C or Ctrl-D, or just throw
 your computer to the ground and storm off.
@@ -80,10 +80,10 @@ your favorite text editor and paste this into it:
           y0 = y1
           iter = iter + 1
         }
-        IO.write(" .-:;+=xX$& "[iter])
+        System.write(" .-:;+=xX$& "[iter])
       }
 
-      IO.print("")
+      System.print("")
     }
 
 Now run: