Rename Public/SecretKey into Note-Public/SecretKey and Public/SecretSpendKey into Public/SecretKey

[ureeves]

Summary

We want to do a rename of the PublicKey and SecretKey structures to NotePublicKey and NoteSecretKey, respectively, as well as rename the PublicSpendKey and SecretSpendKey to PublicKey and SecretKey, respectively.
This is to better reflect their usage in our transaction model, and avoid any confusion when comparing documentation with this code.

Possible solution design or implementation

N/A

Additional context

The ideal situation would be that this repository would be independent of phoenix. This is, however, already not the case, since we use words like "Spend" and offer functionality such as SecretSpendKey::sk_r, which are clearly designed for use by phoenix.

[javiersilva0]

See this document for more details.