[Feature Request] `BytesMut::extend_from_within` (append a slice of itself)

[thanhminhmr]

Consider adding extend_from_within to BytesMut, similar to Vec::extend_from_within.

Use case: append data that already exists in the buffer (e.g., duplicating a prefix/suffix for framing, rolling buffers, backreferences).

Current workaround: put_bytes to add placeholder bytes, then copy_within to overwrite them. This touches memory twice and is inefficient in hot paths.

Proposed API: Similar to Vec::extend_from_within.

impl BytesMut {
    pub fn extend_from_within<R>(&mut self, src: R)
    where
        R: std::ops::RangeBounds<usize>;
}

Expected behavior: src is evaluated against the original buffer (like Vec::extend_from_within); out-of-bounds handling consistent with existing methods.

If this approach is acceptable, I’m happy to follow up with a PR—please let me know your preferred signature/semantics. Thanks for the great effort on a great crate!

[Darksonn]

Since the API already exists on Vec on stable, I'm ok with adding this.