Turbopack: uncell AstPath (#75695)
- Remove the last remaining `AstPath` cells
- Also box a rare `CodeGen` enum variant (brings down enum size from 128 bytes to 48 bytes)
- `code_gens.shrink_to_fit()`
Might help memory slightly, but it's bordering on measurement noise:
```
testing against 0293c96cf32
canary b6f8743ecd
13,75 GB
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 507.86s user 77.80s system 847% cpu 1:09.11 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 503.13s user 77.86s system 855% cpu 1:07.90 total
uncell AstPath 3ec676fea5
13,7 GB
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 505.04s user 83.92s system 853% cpu 1:09.00 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 509.03s user 84.26s system 829% cpu 1:11.53 total
```
