multiple c bindgen issues

## Summary
This issue buckets several problems in `wit-bindgen c` to avoid opening many separate issues at once.

Each sub-item is independently reproducible and either:
- triggers a panic, or
- exhibits timeout (potential DoS).

If you prefer, I can split this bucket into separate issues.

## Environment
- wit-bindgen-cli: 0.47.0
- OS: Ubuntu 22.04.5 LTS (x86_64)

## Sub-issues (details in comments)
1. Slowdown on deep `option<...>` type aliases.
2. Panic on `flags` with >64 fields.
3. `world` import/export same name triggers duplicate symbol panic.
4. Resource method vs free-function name collision triggers duplicate symbol panic.
5. Semver sanitization collision (`-` vs `+`) triggers duplicate symbol panic.
6. Very deep direct `option<...>` nesting causes hang (even stack overflow).