This PR changes how sparse case expressions represent the
none-of-the-above information. Instead of of many `x.ctorIdx ≠ i`
hypotheses, it introduces a single `Nat.hasNotBit mask x.ctorIdx`
hypothesis which compresses that information into a bitmask. This avoids
a quadratic overhead during splitter generation, where all n assumptions
would be refined through `.subst` and `.cases` constructions for all n
assumption of the splitter alternative.
The definition of `Nat.hasNotBit` uses `Nat.rightShift` which is fiddly
to get to reduce well, especially on open terms and with `Meta.whnf`.
Some experimentation was needed to find proof terms that work, these are
all put together in the `Lean.Meta.HasNotBit` module.
Fixes#11183
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Co-authored-by: Rob23oba <152706811+Rob23oba@users.noreply.github.com>
