Implement a trie data structure. Specifically, implement

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

unittests/Basic/EncodedSequenceTest.cpp

@@ -3,6 +3,8 @@
 
 using namespace swift;
 
+namespace {
+
 struct Element {
   unsigned X, Y, Z;
 
@@ -100,6 +102,8 @@ struct Tester {
   }
 };
 
+}
+
 TEST(EncodedSequenceTest, PushIterate) {
   Tester tester;
   tester.check();