Introduce normalizeWithM for monadic normalization

[alexbiehl]

See the discussion in dhall-lang/dhall-lang#133.

[alexbiehl]

• This needs a big comment saying that this might violate assumptions

[Gabriella439]

Don't forget to export NormalizerM and normalizeWithM from the module

[Gabriella439]

I'll close this for now since from offline discussions it sounded like you could do this using the existing API by post-processing the abstract syntax tree to monadically normalize uninterpreted free variables

[ocharles]

I'd like to re-open this in light of dhallix/nix-derivation#8. There it was determined that the only real option for Dhallix's performance right now is to make derivation a built in, and add an extra step to normalization to avoid blowing up the AST. However, this inherently needs some kind of monadic normalization. I need some way to build up the .drvs that I'm going to write to disk later, to instruct Nix how an output is realised. That could be writer or state, but I can't see a way to do it without adding some effects to normalization.

@Gabriel439 Mind if I resurrect and re-work this PR?

[Gabriella439]

@ocharles: Yeah, it seems reasonable to proceed with this change

[ocharles]

Ok, I think this is good for review.

[Gabriella439]

Thank you, both of you 🙂