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_postopt.rs

@@ -5,7 +5,7 @@
 use cretonne;
 use cretonne::ir::Function;
 use cretonne::print_errors::pretty_error;
-use cton_reader::TestCommand;
+use cretonne_reader::TestCommand;
 use std::borrow::Cow;
 use std::fmt::Write;
 use subtest::{run_filecheck, Context, Result, SubTest};