Performs the following renamings in the vetKD API in accordance with the latest changes in the spec PR:

• derivation_id --> input
  • The name derivation_id often caused confusion and input is a more standard name in the context of key derivation schemes.
• derivation_domain (previously derivation_path) --> context
  • The main use case for the derivation domain/path is to do domain separation, i.e., to specify the context in which the derived keys are to be used. Given this, directly calling it context seems beneficial in that it makes the meaning of the field more clear and intuitive, and thus the API easier to use.
• vetkd_derive_encrypted_key --> vetkd_derive_key
  • Although the fact that the returned key is encrypted is relevant in that it ensures that nodes cannot see the key in clear text, this can be considered an implementation detail. Also, the name vetkd_derive_encrypted_key is somewhat long. In any case, in the returned struct the (single) field is still called encrypted_key, so it is still explicit that the returned key is encrypted.
• encryption_public_key --> transport_public_key
  • Everyone everywhere (in publications, slides, demos, etc.) called this "transport public key". The reason this was not called transport_public_key in the API so far was because the containing API method was called vetkd_derive_encrypted_key and the name encryption_public_key should have made it clear that it is this very public key under which the encrypted key is encrypted. Now that we are removing the part encrypted_ from the API name, this reason is obsolete and we are free to call it transport_public_key.