ed25519: fix edwardsToMontgomery formula; implement edwardsToMontgomeryPriv

[mirceanis]

fixes #31

This fixes the formula used in edwardsToMontgomery.
I also implemented the equivalent transformation for private keys edwardsToMontgomeryPriv and added a bunch of tests.

Is there a reason ed25519-addons.test.js wasn't being run during npm test ? I included it as well.

My IDE rearranged the imports and whitespace to fit prettier rules. If that's an issue I can revert.

[ukstv]

@mirceanis Hahaha, was about to make a PR for exactly the same issue! Discovered wrong formula while trying to replace stablelib dependencies with @noble/curves. You made it first!

[ukstv]

Hey @paulmillr do you think you could address the issue we discovered here so that we could further adopt noble-crypto in DID-related realm?

[paulmillr]

Thanks,

Could you add an alias for edwardsToMontgomery called edwardsToMontgomeryPub? We will keep the old name for backwards compat for now.

I also assume the new priv function is the same as ed25519.utils.getExtendedPublicKey().head? And could be replaced with it?

[paulmillr]

renamed manually