secp256k1_XMD:SHA-256_SSWU_RO_ missing the isogenous generator

[skaunov]

This PR is draft since I need a hint! When hashing the lib checks that the isogenous generator maps correctly onto the target curve generator. The problem is I couldn't find the relevant generator for RFC9380 yet (via a reference, or computationally). I found out that there's a path via the isogeny kernel, but I wonder if somebody could suggest a simpler way to obtain that to continue with this.

Description

Adds hashing to the <curves/secp256k1>.

closes: none - just a development

---

Since this isn't ready for merging yet it seems to me the check-list is ok to postpone, correct me if I'm wrong, pls.

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[skaunov]

@mmaker , I guess you've made most of this crate: could you take a look, pls. (Or tell who's the best person for this now.)