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feat: add constant-time trait bounds (#219)

Currently, the only implementation of the `SecretKey` and `PublicKey`
traits is for Ristretto, where both
[scalars](https://github.com/dalek-cryptography/curve25519-dalek/blob/<a class=hub.com/tari-project/tari-crypto/commit/ba737a379071191158bacfa6d138f6249b12fc09">ba737a379/curve25519-dalek/src/scalar.rs#L296-L300)
and [group
elements](https://github.com/dalek-cryptography/curve25519-dalek/blob/ba737a379071191158bacfa6d138f6249b12fc09/curve25519-dalek/src/ristretto.rs#L822-L826)
use constant-time equality in their underlying `PartialEq`
implementations, and which support the `ConstantTimeEq` trait.

This PR does what it can to encourage the use of constant-time equality
for keys by doing a few things.

First, it requires that any types implementing `SecretKey` or
`PublicKey` also implement `ConstantTimeEq`. Unfortunately, this doesn't
guarantee that their `PartialEq` implementation defaults to this, and it
doesn't appear possible to enforce this at the trait level.

It also sets a good example by manually implementing `PartialEq` on the
Ristretto key types to use their `ConstantTimeEq` implementations. This
isn't strictly necessary, but hopefully helps to indicate best practice.
It also implements `ConstantTimeEq` directly as required by the new
trait bounds.

Finally, it implements `ConstantTimeEq` for `DiffieHellmanSharedSecret`
using the new trait bound, and removes a redundant `Zeroize` trait
bound.

Note that this doesn't actually change the current implementations'
behavior, and therefore incurs no performance hit.

Closes #139.

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