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/CMakeLists.txt b/library/CMakeLists.txt
index 19ba315..bef0dbf 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -19,6 +19,7 @@
     base64.c
     bignum.c
     bignum_new.c
+    bignum_core.c
     camellia.c
     ccm.c
     chacha20.c