Reexport `aead` defined types used in the `ursa` public API

[Marin Veršić]

Description of the problem:

ursa lib depends on aead and uses aead defined types in it's public API like here. This forces the user to depend on aead in their Cargo.toml even though they make no use of aead itself except through ursa. Forcing users to manage dependency of a dependency is a malpractice.

Solution

Either reexport entire aead liib from ursa or reexport just aead types that are exposed in the public API of ursa. In the given example that would be either pub export aead or pub export aead::Error. Type aliases are also welcome

[Aleksandr Petrosyan]

Should be addressed in either #225 or a follow-up.