This PR adds eta reduction to the sym discrimination tree lookup
functions (`getMatch`, `getMatchWithExtra`, `getMatchLoop`). Without
this, expressions like `StateM Nat` that unfold to eta-expanded forms
`(fun α => StateT Nat Id α)` fail to match discrimination tree entries
for the eta-reduced form `(StateT Nat Id)`.
Also optimizes `etaReduce` with an early exit for non-lambda expressions
and removes a redundant `n == 0` check.
Includes a test verifying that `P (StateM Nat)` matches a disc tree
entry for `P (StateT Nat Id)`.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
