Ponycheck: Collections generators

[mfelsche]

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Create more generators for creating collections:

• [x] Set/SetIs
• [x] Map/MapIs
• [ ] persistent collections

[mfelsche]

Also add methods on Generators for easily creating collections of random length.

[mfelsche]

Generators for persistent collections https://stdlib.ponylang.org/collections-persistent--index are missing

[mfelsche]

The current generator methods on the primitive Generators already allow for creating collections of random size within bounds