Remove `impl {De,}Serialize for Hugr`

[aborgna-q]

We should be using envelopes instead.

We should keep the serialize::Versioned definition and add some helper method so that envelopes can still use the json format, but it shouldn't be exposed so directly for Hugr.

We should instead have store/load methods as we do with packages.

This could be a breaking serialization change if we decide to store nested hugrs as envelopes rather than raw hugr jsons.

[doug-q]

I'm not so sure this is a good idea.

It is very convenient to be able to write

#[derive(serde::Serialize)]
struct Foo {
  hugr: Hugr, 
  ...
}

[aborgna-q]

That's exactly what we're trying to prevent :)
People shouldn't rely on the (unstable) json serialization, and use envelopes instead.

If anything, we could have an impl {De}Serialize that encodes an envelope with default config and writes it as a string/bytes into the serializer.

[aborgna-q]

I added a serde_with helper in the PR that should solve your usecase. See #2167.