add an initial radix trie implementation
auto merge of #5205 : thestinger/rust/radix, r=graydon

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 when done in the right direction) 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 splitting 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 the test suite with the other maps so that's why I haven't duplicated all of the existing integer key tests in this file. I'll send in another pull request to deal with that.

Current benchmark numbers against the other map types:

    TreeMap:
     Sequential integers:
      insert: 0.798295
      search: 0.188931
      remove: 0.435923
     Random integers:
      insert: 1.557661
      search: 0.758325
      remove: 1.720527

    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 just wanted the essentials first.