clump: revert append-only behavior for annealments.

This is needed for annealment merging. Sequence list remains append-only.
clump: sort annealment keys

Before, the order of the two keys and start points was arbitrary.
By sorting the input to Clump.add_annealment, comparing two annealments
becomes simpler.
clump: merge consecutive annealments

If multiple annealments, on the same pair of sequences, are adjacent,
then they can be represented as one long annealment.

This code is quite ugly.