add specialized Buf::chunks_vectored for Take

[srijs]

The h2 crate uses Take around the data frames to enforce a maximum frame size, which breaks vectored writes since Take doesn't provide a specialized chunks_vectored that calls the underlying chunks_vectored.

To make this work with truncating individual IoSlices, I needed to resort to a tiny bit of unsafe code. There is a PR open (rust-lang/rust#111277) which will alleviate the need for it, but that hasn't landed yet (so unlikely to be stable any time soon).

An alternative could be to just remove rather than truncate any IoSlice that crosses the limit threshold, which could be accomplished without any unsafe code. However, that might break the following guarantee documented on chunks_vectored: "If chunk_vectored does not fill every entry in dst, then dst is guaranteed to contain all remaining slices in `self."

Happy to go either route, let me know what you think!

[srijs]

Just noticed this is a duplicate of #476. Don't have a strong opinion here, please feel free to review and merge either :)