[Feature Request] Slice mutating functions

[washley]

First of all, thanks for writing this crate!

It would be pretty awesome if the slice types had functions to modify attributes, and these wrote-through to the underlying slice.

Another optimization that could be made here is that, since we know what's changing, you can perform an optimized checksum recalculation (see https://tools.ietf.org/html/rfc1631 section 3.3).

[washley]

Of course now I see the roadmap, and this is on there. I don't know if you prefer to close this or leave it open, I'll leave that up to you.