fix: remove global NatCast (Fin n) instance
#8620
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kmill
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kmill
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This PR removes the `NatCast (Fin n)` global instance (both the direct instance, and the indirect one via `Lean.Grind.Semiring`), as that instance causes causes `x < n` (for `x : Fin k`, `n : Nat`) to be elaborated as `x < ↑n` rather than `↑x < n`, which is undesirable. Note however that in Mathlib this happens anyway!
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Lean 4.21 removes the default Nat-to-Fin coercion[1], which some of our proofs and functions relied on. [1]: leanprover/lean4#8620
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This PR removes the
NatCast (Fin n)global instance (both the direct instance, and the indirect one viaLean.Grind.Semiring), as that instance causes causesx < n(forx : Fin k,n : Nat) to be elaborated asx < ↑nrather than↑x < n, which is undesirable. Note however that in Mathlib this happens anyway!