* Upgrade curve25519-dalek to version 3

* Use a version of curve25519-dalek with an up-to-date packed_simd

This:
- introduces a small [thiserror](https://github.com/dtolnay/thiserror)-powered enum to improve ProofVerifyError's messages,
- refactors point decompression errors into a variant of that enum, thereby suppressing the panics which occur when decompresison fails.
- folds other panics into the Error cases of their enclosing `Result` return

Minor tweaks

limit public APIs