Slimmify BTree by replacing the three Vecs in Node with a manually alloc...

...ated buffer.

Before:

    test btree::map::bench::find_rand_100                      ... bench:        29 ns/iter (+/- 2)
    test btree::map::bench::find_rand_10_000                   ... bench:        83 ns/iter (+/- 6)
    test btree::map::bench::find_seq_100                       ... bench:        30 ns/iter (+/- 1)
    test btree::map::bench::find_seq_10_000                    ... bench:        50 ns/iter (+/- 3)
    test btree::map::bench::insert_rand_100                    ... bench:       186 ns/iter (+/- 30)
    test btree::map::bench::insert_rand_10_000                 ... bench:       377 ns/iter (+/- 8)
    test btree::map::bench::insert_seq_100                     ... bench:       299 ns/iter (+/- 10)
    test btree::map::bench::insert_seq_10_000                  ... bench:       368 ns/iter (+/- 12)
    test btree::map::bench::iter_1000                          ... bench:     20956 ns/iter (+/- 479)
    test btree::map::bench::iter_100000                        ... bench:   2060899 ns/iter (+/- 44325)
    test btree::map::bench::iter_20                            ... bench:       560 ns/iter (+/- 63)

After:

    test btree::map::bench::find_rand_100                      ... bench:        28 ns/iter (+/- 2)
    test btree::map::bench::find_rand_10_000                   ... bench:        74 ns/iter (+/- 3)
    test btree::map::bench::find_seq_100                       ... bench:        31 ns/iter (+/- 0)
    test btree::map::bench::find_seq_10_000                    ... bench:        46 ns/iter (+/- 0)
    test btree::map::bench::insert_rand_100                    ... bench:       141 ns/iter (+/- 1)
    test btree::map::bench::insert_rand_10_000                 ... bench:       273 ns/iter (+/- 12)
    test btree::map::bench::insert_seq_100                     ... bench:       255 ns/iter (+/- 17)
    test btree::map::bench::insert_seq_10_000                  ... bench:       340 ns/iter (+/- 3)
    test btree::map::bench::iter_1000                          ... bench:     21193 ns/iter (+/- 1958)
    test btree::map::bench::iter_100000                        ... bench:   2203599 ns/iter (+/- 100491)
    test btree::map::bench::iter_20                            ... bench:       614 ns/iter (+/- 110)

This code could probably be a fair bit cleaner, but it works.

Part of #18009.
This may cause badness if any of the `clone`s unwind, since there will be destructors run on uninitialised memory. I think a correct way to write this is to iterate over keys values and edges together, clone them all onto stack variables, and only then write them into the new values and update the length.