a ternary tree with a fixed-length per-node inline key buffer.
I plan to use this for metadata path caches, where it's useful to
be able to quickly find the most-derived point along a path that
you've already cached, but it should be useful for other things
in the compiler as well, like function-with-argument-label
lookups and possibly code completion.
This is quite a bit more space-efficient (and somewhat faster)
than doing scans after a lower_bound on a std::map<std::string, T>.
I haven't implemented balancing yet, and I don't need delete at
all for metadata paths, so I don't plan to work on that.
Swift SVN r32453
