"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/modules.markdown

@@ -173,7 +173,7 @@ like:
     import "shared"
 
     // shared.wren
-    IO.print("Shared!")
+    System.print("Shared!")
 
 Here, "a" and "b" both want to use "shared". If "shared" defines some top-level
 state, we only want a single copy of that in memory. To handle this, a module's
@@ -211,14 +211,14 @@ For example:
     import "a"
 
     // a.wren
-    IO.print("start a")
+    System.print("start a")
     import "b"
-    IO.print("end a")
+    System.print("end a")
 
     // b.wren
-    IO.print("start b")
+    System.print("start b")
     import "a"
-    IO.print("end b")
+    System.print("end b")
 
 This program runs successfully and prints: