This also adds the 'ignore_degrees' parameter.

This closes #238

This avoids unnecessary copying of the filter property map, which
although being O(1), does end up consuming a measurable time.

This also transforms a run-time exception into a static_assert, in case
there is no available default empty or deleted key of the specified type.

This implements a general wrapper for dense_hash_map and dense_hash_set,
that falls back to unordered_map and unordered_set, if sparsehash is not
enabled.