add an initial radix trie implementation

This is an implementation of a map and set for integer keys. It's an ordered container (by byte order, which is sorted order for integers and byte strings) with O(1) worst-case lookup, removal and insertion. There's no rebalancing or rehashing so it's actually O(1) without amortizing any costs.

The fanout can be adjusted in multiples of 2 from 2-ary through 256-ary, but it's hardcoded at 16-ary because there isn't a way to expose that in the type system yet. To keep things simple, it also only allows `uint` keys, but later I'll expand it to all the built-in integer types and byte arrays.

There's quite a bit of room for performance improvement, along with the boost that will come with dropping the headers on `Owned` `~` and getting rid of the overhead from the stack switches to the allocator. It currently does suffix compression for a single node and then splits into two n-ary trie nodes, which could be replaced with an array for at least 4-8 suffixes before bursting it. There's also the option of doing path compression, which may be a good or a bad idea and depends a lot on the data stored.

I want to share a test suite with the other maps so that's why I haven't duplicated all of the existing integer key tests in this file.

Current benchmark numbers:

    LinearMap:
     Sequential integers:
      insert: 0.272338
      search: 0.141179
      remove: 0.190273
     Random integers:
      insert: 0.293588
      search: 0.162677
      remove: 0.206142

    TrieMap:
     Sequential integers:
      insert: 0.0901
      search: 0.012223
      remove: 0.084139
     Random integers:
      insert: 0.392719
      search: 0.261632
      remove: 0.470401

@graydon is using an earlier version of this for the garbage collection implementation, so that's why I added this to libcore. I left out the `next` and `prev` methods *for now* because I think iterators in the same style as `TreeMap` would be better.