Introduce Cursor and CursorMut traits

[mhebant]

Has many others before, I've been looking for a way to read or write ahead, or seek into a Buf or a BufMut (see #823, #658, #382 to name a few)

While this PR might not solve everyone's problems, I has the benefit of not braking the API (it only adds 2 new traits) and simple (by relying as much as possible on the existing Buf and BufMut).

The simple solution to read ahead in a Buf I've seen proposed (like here #382 (comment)) is to clone the Buf. But making T: Buf + Clone your interface wont work in practice: it's fine if your Buf is a &[u8] but not if it's a Vec<u8>.

So the Idea is to add a trait (named Cursor for now) whit an associated type that is a cheap version of our Buf.
For example the Cursor of a Vec<u8> is a &[u8], for a Chain<Vec<u8>, Vec<u8>> it's a Chain<&[u8], &[u8]> etc...

The Cursor trait allows you to get a cheap copy of you Buf that you can use to read ahead without mutating the original Buf.

This should also work for writing ahead to a BufMut before possibly committing what have been written by calling advance.

This PR is incomplete, lacks some implementation and tests, but I'm looking for feedback before finishing it.

Also I understand adding new features is complicated because any it's impossible to make breaking change. It should be possible to put this in it's own create depending on bytes. But again looking for some feedback.

Thanks !

[Darksonn]

I've spent some time thinking about this. Unfortunately I think the desire to keep bytes simple outweighs this feature request, so I'm going to close it.