Clean up CanonicalName things

I noticed some of the template use in Happly is not ideal. Go back and look closer:

- `CanonicalName<size_t>` looks like it has a bug, perhaps not caught because no one uses 32bit machines?
- Try to avoid copy when converting to canonical
- Several places where we through runtime_errors could be compile time checks (though the compile checks might have much more confusing error message...)