refactor: redefine unsigned fixed width integers in terms of BitVec

[hargoniX]

I made a few choices so far that can probably be discussed:

• got rid of modn on UInt, nobody seems to use it apart from the definition of shift which can use normal mod
• removed the previous defeq optimized definition of USize.size in favor for a normal one. The motivation was to allow OfNat to work which doesn't seem to be necessary anymore afaict.
• Minimized uses of .val, should we maybe mark it deprecated?
• Mostly got rid of .val in basically all theorems as the proper next level of API would now be .toBitVec. We could probably re-prove them but it would be more annoying given the change of definition.
• Did not yet redefine log2 in terms of BitVec as this would require a log2 in BitVec as well, do we want this?
• I added a couple of theorems around the relation of < on UInt and Nat. These were previously not needed because defeq was used all over the place to save us. I did not yet generalize these to all types as I wasn't sure if they are the appropriate lemma that we want to have.

[leanprover-community-mathlib4-bot]

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[kim-em]

Looks great.

Could you leave a deprecation in place for modn, just in case there is a downstream user?

Agree to marking .val as deprecated, but it can also happen later.

BitVec.log2 sounds good, but probably deserves a different name, and can happen later.

Would you mind rebasing onto nightly-with-mathlib so we can check to see how much we have disturbed downstream. (Ideally nothing, but I'm be pleasantly surprised if that were the case.)

[kim-em]

(I'm keen to have this in, as I'm thinking about Fin refactors.)