add std,alloc feature flag to support no_std build

[JyJyJcr]

Proposed changes

add features std, alloc to prepare for no_std build support.

• alloc feature means memory allocation used. Implementation differs depending on std feature:
  • with std: use std crate components. (e.g. std::string::String)
  • without std: use alloc crate components. (e.g. alloc::string::String)
• std feature means std crate components used. Currently the only difference between std and alloc is impl of std::error::Error (core::error::Error is not in Rust 1.79.0), which is not used.

alloc is the transitional feature: while it enables no_std build, it allows memory allocation using std::alloc::System internally. We can gradually replace alloc crate components by nginx-based ones.

For these features, many std imports are replaced by core ones.

Related issue (enhancement): #109

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• I have added tests (when possible) that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[bavshin-f5]

Thanks!
Looks similar to what I intended to do, I'll take a closer look next week when I'm back from vacation.

One comment: I'd prefer to remove String from nginx-sys completely instead of introducing the alloc feature.
Display impl commit in #113 supposed to be a first step towards that, another step is to rewrite nginx_sys::add_to_ngx_table like in bavshin-f5@ae2ee99, and the last step is to remove remaining redundant conversions.

[JyJyJcr]

Thank you for the advice.
I replaced them by your implementation bavshin-f5/ngx-rust@3f054e3, bavshin-f5/ngx-rust@ae2ee99 for the moment.
nginx-sys is now alloc-free.

[bavshin-f5]

The comments for Request::headers_in_iterator/Request::headers_out_iterator need to be updated.

---

Please, squash all the fixup commits and clean up the history.
I believe this PR can be reorganized into 3-4 commits with distinct logical changes. For example,

• Replacing std with core and alloc
• Rework of the header iterator
• Removal of the remaining String uses
• Feature flags and documentation

[JyJyJcr]

Thank you for the reviewing.

I'll fix the points one by one, and after that do squash.

[JyJyJcr]

chores:

• NGINX part of workflows has been broken in some point. It seems unrelated to this PR.
• I've added thumbs up emoji to mark as completed (in my perspective), avoiding annoying you by many short messages.

[bavshin-f5]

* NGINX part of workflows has been broken in some point. It seems unrelated to this PR.

Worker processes crash when running awssig example code:

# got: '2025/01/02 13:13:30 [alert] 5687#5687: worker process 5688 exited on signal 6 (core dumped)'

That's the only example that uses the header iterators, the crash is likely in Iterator::next implementation.

[JyJyJcr]

commit squashed.

[bavshin-f5]

Almost ready, just a couple more issues.

98cb85d and 0174f10 fail to build. Please, consider reordering the commits as shown below to make each commit buildable:

1. fix: refactor NgxListIterator to remove String usage
2. fix: replace other String usage by bavshin-f5's impl
   (use alloc::string::ToString addition in the test code should be moved to the no_std commit)
3. chore: add Cargo features
4. fix: replace std to core and add config out