refactor!: remove unnecessary declarative macros

[bavshin-f5]

Remove a few overly direct translations of C macros to Rust declarative macros.

ngx_string! will be addressed separately, as the replacement has to ensure that the byte slice constants are NULL-terminated, and I haven't figured out the best way to enforce that (fn from_static(src: &'static CStr) -> ngx_str_t is the leading contender).

[xeioex]

There are no nginx core APIs that take a list of variables, terminated
by an empty entry. The variables are always added from the module code,
where we should use Rust iterators.

1. We do not need the comma in the first sentence.
2. The variables -> Variables (not specific variables).

There are no nginx core APIs that take a list of variables terminated
by an empty entry. Variables are always added from the module code,
where we should use Rust iterators.

[xeioex]

Otherwise looks good.