Iterator/Slice improvements

[cehteh]

Turns out that I'll need a IterMut as well, thus I started with some cleanup and implementing the missing bits on Slice iterators. WIP still.

Done:

• DoubleEndedIterator for SliceIter/SegmentedIter
• Factored the Slice stuff into slice.rs

Upcoming:

• The same for *Mut variants
• Factor the iterators into a 'iter.rs'
• Likely: remove SegVec::Iter, rename SliceIter to just Iter, make Slice::iter() use that. That simplifies the Codebase with little to no performance impact. (Same for the *Mut) variants.

[cehteh]

Mostly finished, needs some (self-)review and cleanup.

Slice iterators (immutable and mutable) are now apparently faster than std Vec iterators. They are still slightly slower than SegVec Iter (prolly because the size_hint accounting?).

[cehteh]

Rebased on master, the 'FromIter' are not part of this as noted in https://github.com/mccolljr/segvec/pull/26#issuecomment-1620346517 I 'Ready for merge' this now without both 'From/FromIter' bits, pending the decision for later and another PR.