refactor(eddsa-poseidon)!: restrict message types

[f3r10]

Description

The type supported for the EdDSA message has been reduced to BigNumber (either bigint or string which must still be a stringified bigint))

Related Issue(s)

Closes #230

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] I have run yarn style without getting any errors
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes