Make all byte representations of types comparable by their unsigned byte representation only

This could be a painful change, but is necessary for implementing a trie-based index, and settling for less would be suboptimal; it also should make comparisons cheaper all-round, and since comparison operations are pretty much the majority of C*'s business, this should be easily felt (see CASSANDRA-6553 and CASSANDRA-6934 for an example of some minor changes with major performance impacts). No copying/special casing/slicing should mean fewer opportunities to introduce performance regressions as well.

Since I have slated for 3.0 a lot of non-backwards-compatible sstable changes, hopefully this shouldn't be too much more of a burden.