Locally nameless STLC

[chenson2018]

This PR adds simple typing derivations for locally nameless terms, with proofs of progress and preservation.

I have also reversed the directory structure of LambdaCalculus to be the representation then the system, e.g. {LocallyNameless,Named}/{Untyped,STLC}. This matches the namespaces we have been using and better groups together related code.

[chenson2018]

After this is merged, while we don't have any other PRs pending, I'm going to address all the outstanding lints so we can start with a clean slate.

Could you take a look at the YAML for the lints workflow? I think there is an issue as I'm getting emails that say .github/workflows/lean_lint_suggest.yml: no jobs were run and it's not appearing here on the PR.

[chenson2018]

Thanks for fixing the lint YAML file, seems to be running now. Looking at the output however, it is not catching the lint errors I see locally. I see these messages on the latest run:

warning: nolints file could not be read; treating as empty: scripts/nolints-style.txt
error: `print-style-error.sh` exited with code 255
could not execute external process './scripts/print-style-errors.sh'

[fmontesi]

Thanks for fixing the lint YAML file, seems to be running now. Looking at the output however, it is not catching the lint errors I see locally. I see these messages on the latest run:

warning: nolints file could not be read; treating as empty: scripts/nolints-style.txt
error: `print-style-error.sh` exited with code 255
could not execute external process './scripts/print-style-errors.sh'

Maybe it's got something to do with the version? I'm now using the same they use in mathlib. Otherwise, it might be that we should provide that script?..