BytesMut::unsplit: rename to join

[vorot93]

The naming of unsplit is unfortunate because it can both revert a split and degenerate to extend_from_slice.

Therefore I think that join is a better name that truly reflects what it is.

Also we could consider reporting an error if the slices are not contiguous - this could be an even better alternative to this PR.

[carllerche]

Thoughts cc @tokio-rs/maintainers

[Darksonn]

Do we have a method that only succeeds if they can be joined without copying? If so, having both could make sense, and I do think that unsplit is a good name for the fallible one.

[seanmonstar]

Having a try_unsplit and unsplit (unsplit_at_all_costs) makes sense to me.

I think the name join might conflict (at least mentally) with joining a vec or array of strings with the argument as the glue between each item.

[Matthias247]

Having a try_unsplit and unsplit (unsplit_at_all_costs) makes sense to me.

I agree. A pure unsplit fallible unsplit method would have made sense to me too. But since the non-failing version already exists I think it should just be kept and an the try_unsplit be added.

[Darksonn]

I think unsplit and try_unsplit are actually pretty good.