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>
diff --git a/library/Makefile b/library/Makefile
index ac8f331..a1c4bb9 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -84,6 +84,7 @@
 	     base64.o \
 	     bignum.o \
 	     bignum_new.o \
+	     bignum_core.o \
 	     camellia.o \
 	     ccm.o \
 	     chacha20.o \