Password: replace equals() with verify()

Password: replace equals() with verify()

So far, our key derivation code assumed that it has control over
the salt used by the derivation routines, however I want to add Argon2
support and it doesn't work this way: password_hash() generates the
salt itself, and the only way to verify a password is by using
password_verify(). Current way the things are done doesn't support it
because it relies on the result of password hashing with parameters we
provide to be deterministic.

Therefore, I'm deprecating Password::equals(), as well as whole concept
of comparing Password objects - it's used only in tests anyway. It's
getting replaced with verify() that only accepts password strings.
Uses of old function are fixed with exception of a few calls in tests
that will be addressed in my Argon2 patch.

Change-Id: I2b2be9a422ee0f773490eac316ad81505c3f8571