Merge frontend improvements

[Nadrieril]

These are the changes I made to the frontend for the purposes of Charon over the last few weeks. In rough order, the changes looks like:

• Add an explicit Self: Trait clause to trait methods and consts (replaces Add an explicit Self: Trait clause to trait assoc items #1541);
• Fix ImplExpr::Builtin that had some type errors (replaces Fix ImplExpr::Builtin #1543);
• Improve the translation of Drop information;
• Add variance information to type parameters (replaces Add variance information to generic parameters #1547);
• Cleanup the State infrastructure a little bit (this is opinionated, you might disagree);
• Add information about the metadata to use in unsize coercions;
• Resolve dyn Trait predicates;
• Many improvements to FullDef;
• Add infrastructure to get a monomorphized FullDef; this is used in charon to monomorphize a crate graph.

I fixed import_thir.ml but not all the engine tests pass. I'll leave that to you.

The branch consists of a merge of about 10 branches that each could have been a PR; you can review merge-commit-by-merge-commit if you want, the bullet points above would be roughly the PR titles.

[W95Psp]

Wow, thank you, that's a lot of work!
I will take some time to do some reviews tomorrow morning, not sure I'll be able to go through everything though ^^'