impl Copy for enums

Copy trait is no longer implicit
Update to Rust master

Unboxed closures and type lifetimes
Fixed shadowed lifetimes in lib/codegen.rs, lib/stream.rs, lib/maybe_owned_slice.rs
Convert generated code into rt

It reduces generated code size and simplifies codegen.
Fix compilation

Follow changes in rust master
Move read repeated message boilerplate to rt module

It simplifies codegen and reduces generated file size.
Fix compilation