Ponycheck: allow creating non-empty Sets, Seqs and Maps with corresponding Generator combinators

[mfelsche]

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The collection Generator combinators

• Generators.seq_of
• Generators.set_of
• Generators.set_is_of
• Generators.map_of
• Generators.map_is_of

And any other that are implemented later should allow to create non-empty collections either via flag or a whole other method on Generators.