Extract functions declared in bignum_core.h into a source file with a
matching name.
We are doing this because:
- This is a general best practice/convention
- We hope that this will make resolving merge conflicts in the future
easier
- Having them in a unified source file is a premature optimisation at
this point
Signed-off-by: Janos Follath <janos.follath@arm.com>
