Hash table resizing

[JanWielemaker]

Keri's algorithm for resizing the atom table is way cleaner with less time and space overhead than the one used in the RDF library. Rewriting most likely also fixes the stability issues that this library suffers from.

[JanWielemaker]

Does not work, at least not this easy. The key is that in the atom lookup there is always exactly one answer. Here we have multiple, often many. This means that if we detect a resize happened we have to restart, generating in part the same results.