Implement bounded iterators for BTree

Part of collections reform v1, #18424
Also, iteration is simplified:
```
before
test btree::map::bench::iter_1000                          ... bench:     17177 ns/iter (+/- 6302)
test btree::map::bench::iter_100000                        ... bench:   1735731 ns/iter (+/- 23908)
test btree::map::bench::iter_20                            ... bench:       386 ns/iter (+/- 148)
after
test btree::map::bench::iter_1000                          ... bench:     15777 ns/iter (+/- 346)
test btree::map::bench::iter_100000                        ... bench:   1602604 ns/iter (+/- 73629)
test btree::map::bench::iter_20                            ... bench:       339 ns/iter (+/- 91)
```
cc @gereeter @cgaebel
r? @Gankro
Trying to understand this: basically you make a subslice, and if it contains a single edge you conclude that you must descend deeper? Otherwise (including the case where it's empty?), this must be the LCA?

As always: would love some more docs as to what's going on here.