Add alpha equivalence

[fizruk]

This PR contributes two flavours of $\alpha$-equivalence for foil terms:

1. (alphaEquivRefreshed) First approach is to compare terms after normalizing the binders by renaming each binder to the minimum possible one in the current scope (note that non-minimal binders may occur during substitution).
2. (alphaEquiv) Second approach is to compare terms directly and only rename binders when they disagree. Moreover, renaming only happens for one of the terms (the one with a bigger binder). This relies on a new function added to the foil, called unifyNameBinders, as well as liftRM, a relative analogue of liftM, instead of using substitute.

The second approach is the cleaned up (yet at the moment less efficient due to liftRM) version of unsafeAeq that we have used in the benchmarks: https://github.com/KarinaTyulebaeva/lambda-n-ways/blob/09a995f9644f6bf29db1803eef004ddd382b823d/lib/FreeScoped/Foil.hs#L336-L401

What remains in this PR:

• Implement and use alpha equivalence in Language.LambdaPi.Impl.Foil.
• Derive and use alpha equivalence with TH or GHC.Generics in Language.LambdaPi.Impl.FoilTH.
• Use alpha equivalence in Language.LambdaPi.Impl.FreeFoil.
• Add tests for correctness of both approaches.