Doc BTree better, and add some iteration benches

I previously avoided `#[inline]`ing anything assuming someone would come in and explain to me where this would be appropriate. Apparently no one *really* knows, so I'll just go the opposite way an inline everything assuming someone will come in and yell at me that such-and-such shouldn't be `#[inline]`.

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For posterity, iteration comparisons:

```
test btree::map::bench::iter_20                            ... bench:       971 ns/iter (+/- 30)
test btree::map::bench::iter_1000                          ... bench:     29445 ns/iter (+/- 480)
test btree::map::bench::iter_100000                        ... bench:   2929035 ns/iter (+/- 21551)

test treemap::bench::iter_20                               ... bench:       530 ns/iter (+/- 66)
test treemap::bench::iter_1000                             ... bench:     26287 ns/iter (+/- 825)
test treemap::bench::iter_100000                           ... bench:   7650084 ns/iter (+/- 356711)

test trie::bench_map::iter_20                              ... bench:       646 ns/iter (+/- 265)
test trie::bench_map::iter_1000                            ... bench:     43556 ns/iter (+/- 5014)
test trie::bench_map::iter_100000                          ... bench:  12988002 ns/iter (+/- 139676)
```

As you can see `btree` "scales" much better than `treemap`. `triemap` scales quite poorly.

Note that *completely* different results are given if the elements are inserted in order from the range [0, size]. In particular, TrieMap *completely* dominates in the sorted case. This suggests adding benches for both might be worthwhile. However unsorted is *probably* the more "normal" case, so I consider this "good enough" for now.
The proposal in the collections reform RFC is `swap` would be renamed `insert`