Export tweakAdd and tweakMul for public & private key

[andkom]

• I'm submitting a ...
  [ ] bug report
  [x ] feature request
  [ ] question about the decisions made in the repository
  [ ] question about how to use this project

• Summary

Need this methods to fast calculate incrementing key pair.

[bitjson]

Hey @andkom – thanks for opening an issue! It may be a long while before I can prioritize adding this myself, but I'd love to take a pull request to add those methods to the Secp256k1 interfaces.

In case it's helpful, here's a recent pull request that added new functionality for Secp256k1: bitjson#13

And a few comments that might be a helpful starting point for anyone who wants to work on this: bitjson#11 (comment), bitjson#12 (comment)