Module for_fidl_message_crate

This belongs in the //src/lib/fidl/rust/fidl_message crate, but has to go here to work around the “upstream crates may add a new impl of trait” error https://doc.rust-lang.org/error_codes/E0119.html.

Traits

• Body
  Implementation of fidl_message::Body.
• ErrorType
  Implementation of fidl_message::ErrorType.