require Zvkt in RVA23

[oconnor663]

Closes #133. There hasn't been any discussion there, so I don't know if this change is actually wanted, but I wanted to make it as easy as possible for someone to click the merge button if so :) Here's a copy of the body of that issue:

RVA23 currently mandates both Zkt and V, so it seems natural that it should also mandate Zvkt. Although the optional Zvkng and Zvksg extensions both imply Zvkt, high-performance implementations of a lot of software-oriented cryptography primitives (like ChaCha and Poly1305) depend only on V and Zvbb, and those implementations are likely to assume Zvkt even if it's not officially required.

[aswaterman]

I'm pretty sure this is what's intended, so I'll go ahead and merge it. If there are dissenting voices, feel free to open a new issue.