Switch immutables.Map and pyrsistent.PMap to constantdict

[matthiasdiener]

This currently a proof-of-concept.

Edit: I removed the previous performance results, they were likely caused by some kind of caching of kernels.

TODOs:

• add mutate support to constantdict (and perhaps immutabledict) (or change the single mutate call)
• Change from constantdict import constantdict as Map => from constantdict import constantdict
• Perhaps remove sorted calls (likely in a separate PR)
• This needs a new pytools release
• Benchmark this

Please squash

[matthiasdiener]

This is ready for a first look @inducer. It will need a new pytools release for the CI.

[inducer]

I think this generally looks OK. Flagged some minor wrinkles.

[inducer]

Please don't remove the sorted() calls, FWIW.