We have a private function that makes use of the `Hash` trait to
generically hash map entries. This usage makes patching the `hashes`
module difficult. We can achieve the same thing by using a macro and
passing in the concrete type.

This is an internal change, no effect on logic or public API.

Clean up the test imports in the `p2p` module:

- Use `use super::*` as is conventional.
- Use `sha256d::Hash` as is conventional.

Refactor, no logic changes.

In preparation for feature gating hex stuff pull the bytelike traits
that depend on `hex` (ie, stringly stuff) out into a separate macro and
call the new macro from the original.

Structural change only, no behavioural change.

In preparation for feature gating a bunch of the `hmac` unit tests move
the slicey (`AsRef`, `Borrow`, etc) tests so that all the display
related tests are together.

Code move only, no other changes.

This import is not needed, not sure why its not being flagged by
`clippy`.

No logic change.

In general smaller unit tests are better, and ones that have one assert
and the "best" - or so they say.

In this instance, done mainly in preparation for feature gating one of
the tests in upcoming work.

Refactor only, no test coverage change.

Make the `hex` dependency optional and feature gate code on "hex" as
required.