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/runone.rs

@@ -6,8 +6,8 @@ use cretonne::print_errors::pretty_verifier_error;
 use cretonne::settings::Flags;
 use cretonne::timing;
 use cretonne::verify_function;
-use cton_reader::IsaSpec;
-use cton_reader::parse_test;
+use cretonne_reader::IsaSpec;
+use cretonne_reader::parse_test;
 use std::borrow::Cow;
 use std::fs;
 use std::io::{self, Read};