adjust generator

[oxe-i]

Changes:

1. Replace all occurrences of import Lean with import Lean.Data.Json. Generation of test files is more than 10s faster now.
2. Create 2 helper functions in Helper.lean, to serialize lists and objects as lists. They take a json, a serializing function for each item and a separator to insert between items.
3. Add a generator for dominoes. It returns a test file exactly equal to the one already there.
4. Change the generators for house and relative-distance, to make use of newly inserted helpers and improve readability of their test files.

Given that the structure of the test files is the same, I "hardcoded" a standard indent, similar to the one used in dominoes. If we need another format, I can add an extra parameter to those helper functions.

I'll add diamond next to test the new functions further.

[github-actions]

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