Derive forward attribute

[limemloh]

Purpose

Related to Concordium/concordium-rust-smart-contracts#307.
Support forward attribute in derive macros for Serial, Deserial, DeserialWithState and SchemaType.
Should be merged before Concordium/concordium-rust-smart-contracts#314

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[abizjak]

Looks solid overall, but I have some questions.

• How much is the module size increase when manually inlining nested enums and deriving serialization, vs. when using forward?

• I am not sure how it is ensured that the forward with cis2 for example ensures that repr is u8.

• It would be good to have examples of how an error is manifested when generating the schema using forward, but when the inner type is not an enum. So we can document this error case that is not caught by the compiler.

[abizjak]

Ok, but if you can provide some data on the size difference (or lack thereof) that would be great.

[limemloh]

For the wCCD example: the schema is unchanged in size when using forward attribute, but it should produce the exact same thing, so this is expected.
The smart contract module itself went from 131.487 kB (on current main) to 130.862 kB using rustc 1.69.0 and command:

cargo concordium build --schema-embed

With no_std the same example is unchanged between main and this branch