[Rust] Extract as much compile-time code as possible into functions, and call them via inlining

[urlyy]

Feature Request

The se/de code for Rust struct and enum is expanded at compile time. Right now, all related methods are placed inside quote!{}, including some functions that don’t actually depend on compile-time variables. This makes IDE hints and debugging quite inconvenient. We should extract part of the code out and mark them with #[inline(always)] for inlining.