Refactor helper macro from construct_keys_callback

Refactor helper macro from construct_keys_callback

When constructing a BlindedPath, utils::construct_blinded_hops uses two
iterators. However, this makes it difficult to pad blinded hops to equal
sizes without allocating a vector or cloning data. Refactor the
construct_keys_callback utility function so that is can be used with an
Iterator with different Item types. This allows using a single Iterator
of tuples while still supporting its use only with pubkeys.