[draft] Support map::Trie#children

[j6k4m8]

Hi @shanecelis — based on #35 I've drafted the following implementation.

I'm a rust noob so would be happy to take feedback on this; in particular, I think my trait constraints are correct, but could definitely use some extra eyes on that!

The traversal code I copied from longest_prefix though it's possible we should instead support an exact match only..?

This code also implements children_values and children_labels, where children_labels can accept a boolean terminals_only flag so that only labels for terminal nodes are returned (i.e., direct descendants) instead of for subtries as well.

[j6k4m8]

Wondering now about the best way to support access to the following;

	labels	values
children	✅
children that are terminals	✅	✅
children that are non-terminals	✅

Should that just be four different functions..? (Five including children that returns a ChildNodeIter) Or is there some more elegant pattern we could use...