feat: BitVec.twoPow and lemmas, toward bitblasting multiplication for LeanSAT (#4417)

We add a new definition `BitVec.twoPow w i` to represent `(1#w <<< i)`.
This expression is used to test bits when building the multiplication
bitblaster.

Patch 1/?, being peeled from https://github.com/opencompl/lean4/pull/6.

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Co-authored-by: Tobias Grosser <github@grosser.es>
3 files changed