Add secp256k1_ec_grind to grind keypairs

[sipa]

Unsure if this should be in upstream, but it may be useful for things like shaving off a few bytes from signatures.

[kallewoof]

Tested ACK 885ea63

This PR is used as is in https://github.com/kallewoof/btcdeb/tree/vanitygen -- I verified that the keypair is valid.

I grinded bc1qkallealmkdwwyuc2tmf5c6hzmlaujq6jl38hpe using this, and sent myself 10k goodly satoshi, which a wallet with the imported private key saw in its balance, and could sign transactions for. "ismine": true also appeared in getaddressinfo output. As Nicolas Dorier would put it, I ate my own (@sipa's, rather) dog food.