Introduce property-based testing for syntax packages

[Ailrun]

Is your feature request related to a problem? Please describe.
Current tests are too limited in terms of their syntactic coverages.

Describe the solution you'd like
Add property-based tests.

Describe alternatives you've considered
Cover all the cases by hand.

[Ailrun]

Where should I place hedgehog generators?
Inside of main syntax packages? or in dedicated packages?
Is it even possible to extract them into such packages?