Add xmss from NIST 800 208

[paocalvi]

I'd like to ask if there is any program to add xmss (xmssmt) in the near future. The only golang NIST compliant implementation that I found is the bwesterb's one. All other ones available are stuck at RFC level (NIST draft) and thus not compliant at least at key generation level. This implementation, on the other hand, has a lot of imports and dependencies, whose absence is one of best features of cloudflare circl. Any chance to bring it "inside"? It would be a splendid and useful addition.

[bwesterb]

Some of the dependencies of bwesterb/go-xmssmt could be removed, but several are essential to deal with the stateful private key safely and efficiently.