spake2: password not passed to memory-hard hash function

Hi,

I noticed that the spake2 crate uses HKDF instead of a memory-hard hash function when converting the password to a scalar: https://github.com/RustCrypto/PAKEs/blob/04ca077f2706fbd4bb5ed903a22bdcd2f20b2b0a/spake2/src/lib.rs#L473

According to the draft specification, as well as this analysis, implementers should use a function like scrypt to slow down brute-force attacks. My guess is that HKDF was used for interoperability with Magic Wormhole's Python implementation, where the one-time nature of passwords means brute force isn't a viable attack. However, this may be a problem for other use cases where a password can be attempted more than once without manual intervention from the other side.

Workarounds include request rate limiting or for callers to send the password through something like scrypt first, using the derived key as input to SPAKE2. However, many users of the library won't know to do this, since the lack of a memory-hard function is not necessarily clear from the documentation.

Start in spake2/src/lib.rs around line 473 and compare the password-to-scalar flow with section 3.1 of the linked SPAKE2 draft. Review the interoperability context from Magic Wormhole's Python implementation and determine how a memory-hard function could be introduced without breaking supported use cases. Done means the password derivation resists repeated brute-force attempts and the relevant behavior is covered by tests.