Avoid duplication in ID module (#3024)

This PR:
- Moves the `#[repr(packed)]` into an `InnerId`, to avoid duplicating
said attribute and to open the door for casting `&Id` to `&Id` in
future.
- Moves the `snowflake` serde module into just `InnerId`.
- Generates the ID newtypes via the macro, cleaning up the module
significantly

There should be no publicly facing difference for this, purely internal
changes.

Author: GnomedDev