Fix inconsistent behavior when passing in user-provided private key

[zkfriendly]

Related Issue(s)

Fixes #799

Checklist

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

Hey @zkfriendly, I already started working on this task. Here the branch: https://github.com/semaphore-protocol/semaphore/tree/fix/identity-generation.

There are some discussions going on about what's the best way to manage the input types. There will probably be some changes related to that in addition to the bugfix.