Convert fficxx as executable with proper domain IDL spec language

[wavewave]

Currently, binding library developer should embed C++ interface as a Haskell data and write a Haskell script to generate code. The data structure for C++ interface is pretty simple, so that we can express it in terms of JSON, YAML or more desirably configurable language like Dhall. Then, we can decouple generation code from data and we make fficxx as a pure executable without need for compiling Haskell code for code generation.

This change can simplify build processes that involves fficxx code generation, so it's a very desirable change.