Bignum: extract bignum_mod_raw.h functions

Extract functions declared in bignum_mod_raw.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>

library/CMakeLists.txt

@@ -20,6 +20,7 @@ set(src_crypto
     bignum.c
     bignum_new.c
     bignum_core.c
+    bignum_mod_raw.c
     camellia.c
     ccm.c
     chacha20.c