add EC Curve secp256k1

[pdtgct]

This PR adds support for Elliptic Curve S256, aka secp256k1. The curve is defined in Standards for Efficient Cryptography, SEC 2: Recommended Elliptic Curve Domain Parameters, Section 2.4.1 and commonly used in Blockchain systems, starting with Bitcoin. See Bitcoin Wiki secp256k1.

Go Version Change

Due to the minimum requirement of the library dependency on github.com/decred/dcrd/dcrec/secp256k1/v4 v4.1.0, this change increases the minimum version of go to 1.17.

Travis CI failures

⚠️ The Travis CI builds are failing because Travis is configured to use Go 1.13. ⚠️

If you could update Travis CI to use Go 1.17 those errors would be resolved.

PR Additions

• a dependency on the library, github.com/decred/dcrd/dcrec/secp256k1/v4 v4.1.0 which is a copyfree license and includes the Name in the Curve params, "secp256k1" which is the same name used by HSMs, such as AWS Cloud HSM, openssl and SoftHSM2.
• the curve asn1.ObjectIdentifier{1, 3, 132, 0, 10} is correct for secp256k1, see oid-info.com

The contents of this PR with secp256k1 have been used in an internal project that used SoftHSM2.

[pdtgct]

I will keep this Pull Request open, but this project does not appear to be currently maintained.

If anyone is interested in the content presented here, I will be working off a Fork, at https://github.com/pdtgct/crypto11.