"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.

test/language/return/after_else.wren

@@ -1,3 +1,3 @@
-IO.print(Fn.new {
+System.print(Fn.new {
   if (false) "no" else return "ok"
 }.call()) // expect: ok

test/language/return/after_if.wren

@@ -1,3 +1,3 @@
-IO.print(Fn.new {
+System.print(Fn.new {
   if (true) return "ok"
 }.call()) // expect: ok

test/language/return/after_while.wren

@@ -1,3 +1,3 @@
-IO.print(Fn.new {
+System.print(Fn.new {
   while (true) return "ok"
 }.call()) // expect: ok

test/language/return/in_function.wren

@@ -1,6 +1,6 @@
 var f = Fn.new {
   return "ok"
-  IO.print("bad")
+  System.print("bad")
 }
 
-IO.print(f.call()) // expect: ok
+System.print(f.call()) // expect: ok

test/language/return/in_method.wren

@@ -3,8 +3,8 @@ class Foo {
 
   method {
     return "ok"
-    IO.print("bad")
+    System.print("bad")
   }
 }
 
-IO.print(Foo.new().method) // expect: ok
+System.print(Foo.new().method) // expect: ok

test/language/return/return_null_if_newline.wren

@@ -1,6 +1,6 @@
 var f = Fn.new {
   return
-  IO.print("bad")
+  System.print("bad")
 }
 
-IO.print(f.call()) // expect: null
+System.print(f.call()) // expect: null