Introduce new files rsa_internal.[ch] for RSA helper functions

This commit splits off the RSA helper functions into separate headers and
compilation units to have a clearer separation of the public RSA interface,
intended to be used by end-users, and the helper functions which are publicly
provided only for the benefit of designers of alternative RSA implementations.

library/CMakeLists.txt

@@ -48,6 +48,7 @@ set(src_crypto
     platform.c
     ripemd160.c
     rsa.c
+    rsa_internal.c
     sha1.c
     sha256.c
     sha512.c