to_json compatibility with Python json

[Waidhoferj]

Updates to_json methods to return str types so that they can be used with Python json

Fixes #85

Changes

• Separated type validation into CompatiblePyType, which validates incoming Python types to ensure they can be represented in the YCRDT.
• Added JsonBuilder, which can append json from any JsonBuildable object to a string buffer.
• Updated tests to cover to_json conversion.
• Changed YArray iterator method to return a PyIterator on eagerly evaluated array contents.

Type Conversion Best Practices

In the past, we used to_json as a conversion to native Python types. In the future, we should use the direct constructor of the type we desire:

python_list = list(y_array)
python_dict = dict(y_map)
python_string = str(y_text)

Note that this is not a deep conversion.

YArray Iterator Update

This addresses an unknown bug associated with the move behavior. While move works correctly when we pull the Any representation directly from the Yrs Array, the iterator from the raw pointer seems to move with the moved element.

doc = YDoc()
    arr = doc.get_array('test')

    # Move 9 to 0
    with doc.begin_transaction() as t:
        arr.delete_range(t, 0, len(arr))
        arr.extend(t, [0,1,2,3,4,5,6,7,8,9])
    doc.transact(lambda t: arr.move_to(t, 9, 0))
    assert arr.to_json() == '[9,0,1,2,3,4,5,6,7,8]' # passes
    assert list(arr) == [9,0,1,2,3,4,5,6,7,8] # fails returning [9.0]

While this eager evaluation isn't ideal, it does match the expected behavior in y-wasm and fixes a significant bug with the move feature.

[davidbrochart]

Thanks @Waidhoferj.
As a side note, could you make PRs from your fork of ypy in the future?

[Waidhoferj]

@davidbrochart Sounds good