This renames `cton_frontend` to `cretonne_frontend` and so on. This fixes the first part of #287.
This crate provides a straightforward way to create a
Cretonne IR function and fill it with
instructions translated from another language. It contains an SSA construction
module that provides convenient methods for translating non-SSA variables into
SSA Cretonne IR values via use_var and def_var calls.