Split signature and digest generation to separate functions

Split signature and digest generation to separate functions
- Similar in spirit to commit 825691afb2a8e2c0b3c6a031950f3c7855dc4c6e,
  these have fairly little in common really.
- Change the function names to force breakage on users (these are exported
  in ABI but not in public headers so "users" are internal uses in
  practise): the automatically generated bits in signature header are
  not signatures, but digests (even size is a checksum of sorts) ...
  and fix the couple of internal uses.