Add RingBuf::as_slices

[bfops]

It would be nice to have something like pub fn as_slices(&'a self) -> (&'a [T], &'a [T]) for bulk access of RingBuf data (e.g. for supplying to OpenGL without lots of glBufferSubData calls and without copying to an intermediate buffer).

[Gankra]

I think this is a pretty good idea. I've also considered the possibility of a potentially more expensive method that ensures that the contents are all contiguous, and then spits out a single slice, so that you can use all the slice utilities. Would that be useful to you?

[Gankra]

CC @aturon

[bfops]

I need to be able to handle the case where it's not contiguous, so would that mean the code ends up looking something like:

match buf.as_slice() {
    // contents are contiguous
    Some(slice) => {
        do_things(slice);
    }
    // contents aren't contiguous
    None => {
        let (slice1, slice2) = buf.as_slices();
        do_things(slice1);
        do_things(slice2);
    }

It would save the work of constructing an extra slice when it's unnecessary, but would it add branching in the rest of the cases? Would it be more or less performant to return something like:

enum RingBufSlice<'a, T> {
    Contiguous(&'a [T]),
    Split((&'a [T], &'a [T])),
}

[aturon]

@gankro I'm happy to see a method like this land.

[Gankra]

I'll add it to the hypothetical collections reform part 2.

[bfops]

Closed in #19903. Thanks everybody!