Merge #845: Extract the secret key from a keypair

33cb3c2b1fc3f3fe46c6d0eab118248ea86c1f06 Add secret key extraction from keypair to constant time tests (Elichai Turkel)
36d9dc1e8e6e3b15d805f04c973a8784a78880f6 Add seckey extraction from keypair to the extrakeys tests (Elichai Turkel)
fc96aa73f5c7f62452847a31821890ff1f72a5a4 Add a function to extract the secretkey from a keypair (Elichai Turkel)

Pull request description:

  With schnorrsig if you need to tweak the secret key (for BIP32) you must use the keypair API to get compatible secret/public keys which you do by calling `secp256k1_keypair_xonly_tweak_add()`, but after that there's no currently a way to extract the secret key back for storage.
  so I added a `secp256k1_keypair_seckey` function to extract the key

ACKs for top commit:
  jonasnick:
    ACK 33cb3c2b1fc3f3fe46c6d0eab118248ea86c1f06
  real-or-random:
    ACK 33cb3c2b1fc3f3fe46c6d0eab118248ea86c1f06 code inspection, tests pass

Tree-SHA512: 11212db38c8b87a87e2dc35c4d6993716867b45215b94b20522b1b3164ca63d4c6bf5192a6bff0e9267b333779cc8164844c56669a94e9be72df9ef025ffcfd4

diff --cc src/modules/extrakeys/main_impl.h
Simple merge

diff --cc src/modules/extrakeys/tests_impl.h
Simple merge

diff --cc src/valgrind_ctime_test.c
Simple merge