Add resize_and_overwrite

[Andersama]

Following feedback from #49 I've written resize_and_overwrite using https://en.cppreference.com/w/cpp/string/basic_string/resize_and_overwrite as a guide.

[martinus]

Thanks, and sorry for the delay. Picked this up in #59, which keeps your commit.

Three things needed changing: op was called with the old size rather than count (the standard passes count, and callbacks need it to know how much space they have), and op throwing caused a double-free when shrinking and leaked every element when growing. #59 fixes those and adds test/unit/resize_and_overwrite.cpp with 11 cases.

Your point in #49 about the signature being confusing still stands, for what it's worth — but I'd rather match the standard exactly than invent a variant with the same signature and different meaning.

[martinus]

Merged as fd04160 via #59 — your commit is in main with your authorship intact. GitHub can't auto-close this because the repo uses rebase-merge, which rewrites SHAs. Thanks for pushing on this across both PRs.