module style 4.5: introduce function cache to speed up some optimizations

[prozacchiwawa]

Note

Medium Risk
Introduces a new caching path in codegen that can change emitted helper bodies when cache keys collide or are computed incorrectly, and it touches core compilation/optimization flow. Risk is mitigated by being opt-in (requires funcache + dependency graph) and covered by new targeted tests.

Overview
Adds an optional per-function codegen cache (Funcache) to reuse previously generated non-inline defun outputs during optimization runs when the helper’s effective dependencies (used constants/inlines + relevant env shape) haven’t changed.

codegen/codegen_ now accept an optional FunctionDependencyGraph; when provided and funcache is enabled on the compile context, helpers can be served from cache via a sha256tree-based key computed from the helper form plus depended-on forms (with unused env entries nil’d). Adds new tests for module disk-cache key stability/try_from_cache behavior and for funcache correctness, plus a small coverage-script doc update and .gitignore additions.

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