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.

lib/filetests/src/test_verifier.rs

@@ -11,7 +11,7 @@
 
 use cretonne::ir::Function;
 use cretonne::verify_function;
-use cton_reader::TestCommand;
+use cretonne_reader::TestCommand;
 use match_directive::match_directive;
 use std::borrow::{Borrow, Cow};
 use subtest::{Context, Result, SubTest};