digest: add newtype! macro

[newpavlov]

TODO: support generic parameters (e.g. output size) on generated newtype

Closes #1069

[baloo]

I'm a bit worried we have to list all the traits in the invocation of the macro and that consumers will drift inconsistent over time.

We can't centrally add/implement a trait (like we could with the CoreWrapper or my set of macros (#1775)).

[newpavlov]

What do you mean by "drift"? The list of traits should not change across semver-compatible releases of digest.

To make the list of traits less annoying, I plan to add several templates for the most common cases. In other words, instead of:

newtype!(
    /// SHA-256 hash
    Sha256(..);
    delegate:
        Debug AlgorithmName
        Clone Default Reset
        BlockSizeUser OutputSizeUser HashMarker
        Update FixedOutput FixedOutputReset
);

We will write:

newtype!(
    /// SHA-256 hash
    Sha256(..);
    delegate_template: FixedOutputHash
);

[baloo]

We've added traits in the previous cycle: #1098 (released in digest 0.10.4)

We may continue to do so in the next one. Having to list the traits means we have to each consumers and bump the list everywhere and release them.
It's less of a problem with a template though.