Ensure only valid keys are used to create nacl.Decrypter

[robdefeo]

Is your feature request related to a problem? Please describe.
With the addition of different public key types, it is possible to create a signer with an incompatible public key

Describe the solution you'd like
Check public key type is valid before creating signer, currently it must be ed25519, return an error if not

Describe alternatives you've considered
N/a

Additional context
crypto/cipher/nacl/decrypt.go NewDecrypter gives an example of how this could be done.

[robdefeo]

Thank you to @Despire for identifying that this check is already performed by this code