Make BTree's internals safer and do more checks at compile time instead of run time

Before:
```
test btree::map::bench::find_rand_100                      ... bench:        12 ns/iter (+/- 0)
test btree::map::bench::find_rand_10_000                   ... bench:        13 ns/iter (+/- 1)
test btree::map::bench::find_seq_100                       ... bench:        11 ns/iter (+/- 0)
test btree::map::bench::find_seq_10_000                    ... bench:        11 ns/iter (+/- 1)
test btree::map::bench::insert_rand_100                    ... bench:       106 ns/iter (+/- 1)
test btree::map::bench::insert_rand_10_000                 ... bench:       326 ns/iter (+/- 8)
test btree::map::bench::insert_seq_100                     ... bench:       198 ns/iter (+/- 1)
test btree::map::bench::insert_seq_10_000                  ... bench:       312 ns/iter (+/- 3)
test btree::map::bench::iter_1000                          ... bench:     16563 ns/iter (+/- 173)
test btree::map::bench::iter_100000                        ... bench:   1686508 ns/iter (+/- 108592)
test btree::map::bench::iter_20                            ... bench:       365 ns/iter (+/- 25)
```

After:
```
test btree::map::bench::find_rand_100                      ... bench:        12 ns/iter (+/- 0)
test btree::map::bench::find_rand_10_000                   ... bench:        12 ns/iter (+/- 0)
test btree::map::bench::find_seq_100                       ... bench:        11 ns/iter (+/- 0)
test btree::map::bench::find_seq_10_000                    ... bench:        11 ns/iter (+/- 0)
test btree::map::bench::insert_rand_100                    ... bench:        89 ns/iter (+/- 1)
test btree::map::bench::insert_rand_10_000                 ... bench:       121 ns/iter (+/- 3)
test btree::map::bench::insert_seq_100                     ... bench:       149 ns/iter (+/- 0)
test btree::map::bench::insert_seq_10_000                  ... bench:       228 ns/iter (+/- 1)
test btree::map::bench::iter_1000                          ... bench:     16965 ns/iter (+/- 220)
test btree::map::bench::iter_100000                        ... bench:   1687836 ns/iter (+/- 18746)
test btree::map::bench::iter_20                            ... bench:       366 ns/iter (+/- 21)
```
That's what I suspected. Great! Looks good to me as-is then.