Make inputs to recv_mac and meta_recv_mac immutable
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Previously, you had to pass a mutable slice to
(meta_)recv_mac. Regardless of the output, the value left in the buffer was the XOR of the expected MAC with the received MAC. This is a pretty leaky abstraction, and one that theoretically allows the user to make their own insecure MAC test by simply checking if the buffer is all-zeros.This PR makes the
(meta_)recv_macinput immutable, and instead makes a copy internally. To ensure a copy is made, it requires a&[u8; N]for some constantN. This is a nontrivial choice for API ergonomics, but the MAC sizes for a system do not usually change, and they certainly won't exceed the quantitiesNthat the useful traits on[u8; N]are defined for.Separately, this PR makes the encryption example more secure by adding nonces and authentication.