Do a real subtree pull of secp256k1 ff4714e6417c56f7235b287d4a9e555df10e09af

The instructions for doing so are pretty straightforward. Add a
secp-zkp remote and fetch from it, then do

git subtree --prefix=src/secp256k1 pull --squash secp-zkp <commit>

There should be no conflicts (although this one shows a conflict in
.gitignore because it seems I forgot to copy that over during the
last "manual" subtree update). Git will create a merge commit and
you should give this a friendlier message which at least gives the
libsecp-zkp commit ID that you're updating to.

Importantly, if you forget the --squash you will get confusing
error messages because git won't understand how to find the branch
it used for the last `subtree pull`.

Author: apoelstra

src/secp256k1/.gitignore

@@ -1,6 +1,9 @@
 bench_inv
 bench_ecdh
 bench_ecmult
+bench_generator
+bench_rangeproof
+bench_schnorrsig
 bench_sign
 bench_verify
 bench_schnorr_verify