secp256k1/ecdsa: Expose r and s value of signature.

[minizilla]

Why

R and S value can be useful for some computation, currently it is impossible to get R and S value from this package since it is unexported.

Closes #3187.

[minizilla]

close #3187

[minizilla]

I don't think unit test for getter is necessary.

[davecgh]

Thanks for the PR!

I don't think unit test for getter is necessary.

It's not strictly necessary, so I won't hold the PR up over it.

I'll probably add one in a separate PR though because I noticed a code branch that isn't being hit while I was checking and I prefer to keep the cover as close to 100% as is possible. There are a couple of cases that are simply impossible to hit because they would require breaking the underlying crypto to compute.

Before:

$ go test -cover
PASS
coverage: 99.2% of statements

After:

$ go test -cover
PASS
coverage: 98.3% of statements

[davecgh]

Do you have a preference for attribution in the overall dcrd release notes for the next release? I typically just use contributor's name displayed on their GitHub profile unless they request otherwise.