Bignum: extract bignum_core.h functions

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>

include/mbedtls/mbedtls_config.h

@@ -2009,7 +2009,9 @@
  *
  * Enable the multi-precision integer library.
  *
- * Module:  library/bignum.c, library/bignum_new.c
+ * Module:  library/bignum.c
+ *          library/bignum_new.c
+ *          library/bignum_core.c
  * Caller:  library/dhm.c
  *          library/ecp.c
  *          library/ecdsa.c