WIP: feat: derive Copy trait for messages where possible

[caspermeijn]

Rust primitive types can be copied by simply copying the bits. Rust structs can also have this property by deriving the Copy trait.

Automatically derive Copy for:

• messages that only have fields with primitive types
• the Rust enum for one-of fields
• messages whose field type are messages that also implement Copy

Generated code for Protobuf enums already derives Copy.

Other commit

fix: fq_message_name should begin with one dot

When package name is empty, but type_path is filled, the fq_message_name should begin with a single dot.

Open issues

Requires #981 to be merged first