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/src/utils.rs

@@ -3,7 +3,7 @@
 use cretonne::isa;
 use cretonne::isa::TargetIsa;
 use cretonne::settings::{self, FlagsOrIsa};
-use cton_reader::{parse_options, Location};
+use cretonne_reader::{parse_options, Location};
 use std::fs::File;
 use std::io::{self, Read};
 use std::path::Path;