Support for (64-bit) unsigned keys/values.

[jamadden]

RelStorage is using LLBTree internally to store some OIDs and transaction IDs. Those are defined as unsigned 64-bit integers, but LLBTree stores signed data. For very large or rapidly changing databases, eventually we'll need to store a value larger than 2^63-1, and then we'll get a ValueError.

So to avoid falling back to a regular dict, I need a BTree type that is either an unsigned 64-bit integer, or something larger (as in #100). I'd prefer unsigned integers because saving memory was one of the primary reasons for using LLBTree in the first place.

[freddrake]

I think it would be fairly reasonable to add 64-bit unsigned long support, adding all or some of the variants UUBTree, ULBTree, UOBTree; I wouldn't imagine other combinations would actually be that useful.

Adding the L variants wasn't too painful, as best I recall (keeping in mind it was some time ago); it should be pretty clear how to do it if you can find the change(s) that introduced those.