"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/assignment/associativity.wren

@@ -4,6 +4,6 @@ var c = "c"
 
 // Assignment is right-associative.
 a = b = c
-IO.print(a) // expect: c
-IO.print(b) // expect: c
-IO.print(c) // expect: c
+System.print(a) // expect: c
+System.print(b) // expect: c
+System.print(c) // expect: c

test/language/assignment/global.wren

@@ -1,8 +1,8 @@
 var a = "before"
-IO.print(a) // expect: before
+System.print(a) // expect: before
 
 a = "after"
-IO.print(a) // expect: after
+System.print(a) // expect: after
 
-IO.print(a = "arg") // expect: arg
-IO.print(a) // expect: arg
+System.print(a = "arg") // expect: arg
+System.print(a) // expect: arg

test/language/assignment/local.wren

@@ -1,10 +1,10 @@
 Fn.new {
   var a = "before"
-  IO.print(a) // expect: before
+  System.print(a) // expect: before
 
   a = "after"
-  IO.print(a) // expect: after
+  System.print(a) // expect: after
 
-  IO.print(a = "arg") // expect: arg
-  IO.print(a) // expect: arg
+  System.print(a = "arg") // expect: arg
+  System.print(a) // expect: arg
 }.call()

test/language/assignment/syntax.wren

@@ -2,10 +2,10 @@
 var a = "a"
 var b = "b"
 a = b = "chain"
-IO.print(a) // expect: chain
-IO.print(a) // expect: chain
+System.print(a) // expect: chain
+System.print(a) // expect: chain
 
 // Assignment on RHS of variable.
 var c = a = "var"
-IO.print(a) // expect: var
-IO.print(c) // expect: var
+System.print(a) // expect: var
+System.print(c) // expect: var