Use frozendict package

[terrorfisch]

Use frozendict package which includes a C extension instead of self-written python code. The frozendict creation is roughly 5 times faster on my machine.

We keep the hand rolled code around until the next release so we do not break code.

TODO:

• DeprecationWarning if frozendict is not installed
• Measure performance influence
• Add newspiece

Performance influence:

• on my machine 3% improvement of

 ppt = PointPT([(0, 'i'), (1, 'i')], ('a',))
 fpt = ForLoopPT(ppt, 'i', 'n')
 fpt.create_program(parameters=dict(n=10000))```