Merge #204

204: Switch all crate names to use underscores rather than hyphens. r=jrvanwhy a=jrvanwhy

We previously had `libtock_codegen` and `libtock-core`, plus `test-runner` and `print-sizes`. Because Rust does not allow hyphens in crate names in code, Cargo automatically translates the hyphens to underscores (e.g. an application refers to items in `libtock-core` using `libtock_core::`). I decided to make the crate naming consistently use underscores rather than hyphens so that crate names in the repository match their use in user code.

Co-authored-by: Johnathan Van Why <jrvanwhy@google.com>