Create specific type for each of the public key types

[benesjan]

Just fixed a bug in Nico's recent PR and I realized that the issue was annoying to spot because we were missing key types check.

Create a specific type for each of the key type and use it all around the codebase.

E.g.:

struct NpkM {
    inner: Point
}
struct IvpkM {
    inner: Point
}
struct IvpkM {
    inner: Point
}
struct TpkM {
    inner: Point
}