`Poly1305::raw_result` copies the output into a slice, for some
reason allowing any length sice. This isn't a great API, so while
we're here we change it to return the 16-byte tag instead.

Rather than skipping compilation of `poly1305.rs` when building for
fuzzing and relying on `ChaCha20Poly1305` to do the fuzzing
variants, implement an actual fuzz wrapper in `poly1305.rs`,
keeping the same fuzz MAC structure that we already have.

We also add a fuzzing implementation of `fixed_time_eq` which does
a simple comparison, to allow the fuzzer to "see into" the
comparison in some cases.

Best reviewed with `-b`.

`ChaChaPolyReadAdapter` decodes an arbitrary object and checks the
poly1305 tag. In the coming commits, we'll need a variant of this
which allows for an *optional* AAD in the poly1305 tag, accepting
either tag as valid, but indicating to the caller whether the AAD
was used.

We could use the actual AAD setup in poly1305, which puts the AAD
first in the MAC (and then pads it out to a multiple of 16 bytes),
but since we're gonna check both with and without, its nice to
instead put the AAD at the end, enabling us to only calculate most
of the hash once before cloning its state and adding the AAD block.

We do this by swapping the AAD and the data being MAC'd in the
AAD-containing MAC check (but leaving them where they belong for
the non-AAD-containing MAC check).

We also add a corresponding `chachapoly_encrypt_with_swapped_aad`
which allows encrypting with the new MAC format.

In the upcoming commit, we introduce the usage of a new `ReceiveAuthKey`
that will be used to authenticate message contexts in the received
`BlindedMessagePath`s.

When we receive an onion message, we often want to make sure it was
sent through a blinded path we constructed. This protects us from
various deanonymization attacks where someone can send a message to
every node on the network until they find us, effectively
unwrapping the blinded path and identifying its recipient.

We generally do so by adding authentication tags to our
`MessageContext` variants. Because the contexts themselves are
encrypted (and MAC'd) to us, we only have to ensure that they
cannot be forged, which is trivially accomplished with a simple
nonce and a MAC covering it.

This logic has ended up being repeated in nearly all of our onion
message handlers, and has gotten quite repetitive.

Instead, here, we simply authenticate the blinded path contexts
using the MAC that's already there, but tweaking it with an
additional secret as the AAD in Poly1305. This prevents forgery as
the secret is now required to make the MAC check pass.

Ultimately this means that no one can ever build a blinded path
which terminates at an LDK node that we'll accept, but over time
we've come to recognize this as a useful property, rather than
something to fight. Here we finally break from the spec fully in
our context encryption (not just the contents thereof).

This will save a bit of space in some of our `MessageContext`s,
though sadly not in the blinded path we include in `Bolt12Offer`s,
so they're generally not in space-sensitive blinded paths.

We can apply the same logic in our blinded payment paths as well,
but we do not do so here.

This commit only adds the required changes to the cryptography, for
now it uses a constant key of `[41; 32]`.

This commit replaces the hardcoded key used for authenticating the
context in incoming `BlindedMessagePath`s with a dedicated
`ReceiveAuthKey`.

This makes the authentication mechanism explicit and configurable
for the user.

Changes include:
- Introducing `ReceiveAuthKey` to the `NodeSigner`, used to authenticate
  the context at the final hop of an incoming blinded path.
- Updating `BlindedMessagePath::new` to accept a `ReceiveAuthKey` as a
  parameter during path construction.

Now that we have introduced an alternate mechanism for authentication
in the codebase, we can safely remove the now redundant (hmac, nonce)
fields from the MessageContext's while maintaining the security of the
onion messages.