Support the new flat JSON ABI file format

[digorithm]

To support generics, Vec, and more, a lot of effort went into redesigning the JSON ABI file format; the discussion can be found here.

We have a couple of hand-made examples already, so both teams (SDK and Compiler) can work on it in parallel.

This issue tracks this support. Tasks:

• Create an alternate path for the abigen to support both file formats while we make the transition;
• In the new path, generate functions and function selectors based on the new format;
• In the new path, generate the custom types based on the new format;
• Run tests with all hand-made examples;
• Once the compiler outputs the new file format, replace the original abigen with the new one and make necessary adjustments.

Once this is done and we have feature parity between the old and new format, we can start adding support for generics and other newer features.