Add check for the message parameter size in the EdDSA Poseidon library

[cedoor]

Describe the improvement you're thinking about

The type of the message parameter in the @zk-kit/eddsa-poseidon library (i.e. BigNumberish) is currently treated similarly to the private key, but the two values are actually different. While the private key can be of arbitrary length as it is then hashed with keccak (re #188), the message cannot be more than 32 bytes. The size of the input must therefore be checked.

Additional context

Originated from discussion in #178 (comment).

[hannahredler]

Hey! I'm a student of the PSE core program. I'd be interested in taking this issue if its available!

[cedoor]

Hi @hannahredler, of course! I'll assign it to you 👍🏽