"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/module/shared_import/a.wren

@@ -1,5 +1,5 @@
 // nontest
-IO.print("a")
+System.print("a")
 import "shared" for Shared
 var A = "a " + Shared
-IO.print("a done")
+System.print("a done")

test/language/module/shared_import/b.wren

@@ -1,5 +1,5 @@
 // nontest
-IO.print("b")
+System.print("b")
 import "shared" for Shared
 var B = "b " + Shared
-IO.print("b done")
+System.print("b done")

test/language/module/shared_import/shared.wren

@@ -1,3 +1,3 @@
 // nontest
-IO.print("shared")
+System.print("shared")
 var Shared = "shared"

test/language/module/shared_import/shared_import.wren

@@ -8,5 +8,5 @@ import "b" for B
 // expect: b
 // expect: b done
 
-IO.print(A) // expect: a shared
-IO.print(B) // expect: b shared
+System.print(A) // expect: a shared
+System.print(B) // expect: b shared