Add reusable zipper buffers

[imlvts]

This introduces ZipperBuffer. This allows re-using zipper path and stack Vecs.
Name up for change, don't have a strong preference.

fn ReadZipper::detach(self) -> ZipperBuffer; // saves path
fn ZipperBuffer::attach(self, &map) -> ReadZipper; // restores path
fn ReadZipper::relocate(self, &map) -> ReadZipper; // same path on a different map

detach + attach, relocate preserve the path. might need to change the API so this is explicit.

A few additional non-public API changes to make it work.
descend_to_byte used instead of descend_to to avoid aliased &mut.

[adamv-symbolica]

Nice! I quite like the API and naming.
The ancestor_node thing is a little ugly, but the lifetime tagging makes sense.
What's the motivation for going for a new object, rather than just a "disabled RZ" which is a zero-overhead opaque-type wrapper of a RZ?