Dilithium pack and unpack

[pnwamk]

A few additional functions that make it easy for
external users to pack and unpack generated
dilithium keys.

[pnwamk]

CI for b9e7146 matches current HEAD:

== Done checking 148 Cryptol files ===
25 cryptol-spec files failed to load:
  ./McEliece_KEM/high-level/Decapsulation.cry
  ./McEliece_KEM/high-level/Encapsulation.cry
  ./McEliece_KEM/high-level/Gauss.cry
  ./McEliece_KEM/high-level/Key_Generation.cry
  ./McEliece_KEM/high-level/Shared.cry
  ./McEliece_KEM/high-level/benes.cry
  ./McEliece_KEM/high-level/decrypt.cry
  ./McEliece_KEM/high-level/decrypt_helpers.cry
  ./McEliece_KEM/high-level/driver.cry
  ./McEliece_KEM/low-level/benes.cry
  ./McEliece_KEM/low-level/decrypt.cry
  ./McEliece_KEM/low-level/decrypt_helpers.cry
  ./McEliece_KEM/low-level/driver.cry
  ./McEliece_KEM/low-level/encrypt.cry
  ./McEliece_KEM/low-level/gf.cry
  ./McEliece_KEM/low-level/operations.cry
  ./McEliece_KEM/low-level/util.cry
  ./McEliece_KEM/spec/Types.cry
  ./Primitive/Asymmetric/Signature/DSA/p1024_sha1.cry
  ./Primitive/Asymmetric/Signature/DSA/p2048_q224.cry
  ./Primitive/Asymmetric/Signature/ECDSA/ECDSA_sign_tests.cry
  ./Primitive/Symmetric/Cipher/Block/KATAN.cry
  ./Primitive/Symmetric/Cipher/Block/SHACAL.cry
  ./Primitive/Symmetric/KDF/HKDF256.cry
  ./Primitive/Symmetric/MAC/HMAC.cry

[ajayeeralla]

The reason I have pack/unpack Pk and Sk that way is to match the reference implementation and I would verify those functions using SAW. I would right correctness property to prove that the wrappers you wrote were equivalent to mine there. Also, you could add comments explaining the motivation to have these wrappers.

Otherwise, looks straightforward!