HashPassword hashes against the current preferred hasher.

VerifyPassword verifies an entered password against a hashed password and returns whether the hash is "stale" (i.e., was verified using the FIRST preferred hasher above).

BcryptPasswordHasher handles password hashing with Bcrypt.

DummyPasswordHasher is for testing ONLY.

PasswordHasher is an interface type to declare a general-purpose password management tool.