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.
Random possibility: have these functions return `Result<EdgeHandler<'a>, ()>` where

```rust
struct EdgeHandler<'a, K, V> { node: &'a mut Node<K, V> }

impl<'a, K, V> EdgeHandler<'a, K, V> {
    fn push_edge(&mut self, edge: Node<K, V>) { ... }

    fn insert_edge(&mut self, edge: Node<K, V> { ... }

    fn pop_edge ...

    fn remove_edge ...
}
```

One would return `Ok(...)` for an internal node and `Err(())` for a leaf. The `Result` ensures the return value is always used and so it hopefully becomes a bit harder to forget to do this. Of course, this may just not be worth it.