Bignum: extract bignum_mod.h functions

Extract functions declared in bignum_mod.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

This makes library/bignum_new.c empty and therefore it is deleted.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 2920c55..96c12fb 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -18,9 +18,9 @@
     asn1write.c
     base64.c
     bignum.c
-    bignum_new.c
     bignum_core.c
     bignum_mod_raw.c
+    bignum_mod.c
     camellia.c
     ccm.c
     chacha20.c