perf: implement `Expr.instantiateLevelParams` natively

This PR implements `Expr.instantiateLevelParams` in C++. It previously ran `Expr.replace` with a Lean closure called at every visited node. Levels containing `max`/`imax`/`succ` over parameters are still instantiated by `Level.instantiateParams`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
3 files changed