Rename 'cton_*' library names to match the 'cretonne_*' crate names.

This renames `cton_frontend` to `cretonne_frontend` and so on.

This fixes the first part of #287.

cranelift/tests/filetests.rs

@@ -1,7 +1,7 @@
-extern crate cton_filetests;
+extern crate cretonne_filetests;
 
 #[test]
 fn filetests() {
     // Run all the filetests in the following directories.
-    cton_filetests::run(false, &["filetests".into(), "docs".into()]).expect("test harness");
+    cretonne_filetests::run(false, &["filetests".into(), "docs".into()]).expect("test harness");
 }