Add function/s to convert BIG_XXX to BIG_YYY

[nmajid2]

Goal

The goal is to facilitate converting BIG_XXX to BIG_YYY by introducing new functions to ff.c.in

Acceptance criteria

1. Well documented code
2. Tests pass with more than 80% test coverage
3. Static analysis should pass
4. Code should be reviewed by two engineers

[giorgiozoppi]

I rather prefer a macro or set of well defined macro. It's ok, provide the code and we'll review it with @kealan