"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/benchmark/binary_trees.wren

@@ -24,10 +24,10 @@ var minDepth = 4
 var maxDepth = 12
 var stretchDepth = maxDepth + 1
 
-var start = IO.clock
+var start = System.clock
 
-IO.print("stretch tree of depth ", stretchDepth, " check: ",
-    Tree.new(0, stretchDepth).check)
+System.print("stretch tree of depth " + stretchDepth.toString + " check: " +
+    Tree.new(0, stretchDepth).check.toString)
 
 var longLivedTree = Tree.new(0, maxDepth)
 
@@ -44,10 +44,12 @@ while (depth < stretchDepth) {
     check = check + Tree.new(i, depth).check + Tree.new(-i, depth).check
   }
 
-  IO.print((iterations * 2), " trees of depth ", depth, " check: ", check)
+  System.print((iterations * 2).toString + " trees of depth " +
+      depth.toString + " check: " + check.toString)
   iterations = iterations / 4
   depth = depth + 2
 }
 
-IO.print("long lived tree of depth ", maxDepth, " check: ", longLivedTree.check)
-IO.print("elapsed: ", (IO.clock - start))
+System.print("long lived tree of depth " + maxDepth.toString + " check: " +
+    longLivedTree.check.toString)
+System.print("elapsed: " + (System.clock - start).toString)