fix: No mangle on cimpl_free

[tmathern]

Changes in this pull request

Remove the no_mangle on cimpl_free:

• so that C symbols won't have it, enabling multiple libraries to use this crate at the same time without symbols collisions;
• c_impl looks like a Rust API anyway, since the implementing Rust C FFI should use it.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.15%. Comparing base (da629e6) to head (d0360e8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   65.15%   65.15%           
=======================================
  Files           2        2           
  Lines         815      815           
=======================================
  Hits          531      531           
  Misses        284      284           

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[gpeacock]

I had to think about this one, but this is correct