Revert "Use sortedcontainers instead of blist"

This reverts commit 9fbe70a7.

It turns out that sortedcontainers.SortedDict is not an exact match for
blist.sorteddict; in particular, `popitem()` removes things from the opposite
end of the dict.

This is trivial to fix, but I want to add some unit tests, and potentially some
more thought about it, before we do so.