make find_distributed_partition deterministic

[matthiasdiener]

Needs https://github.com/matthiasdiener/orderedsets

Needs:

• change isinstance(..., frozenset) to abc_Set, pt2 loopy#825
• a new pytools release

Partially addresses #498

[matthiasdiener]

This is ready for a first look @inducer. For more comprehensive tests, I think we will need #459.