Nested DSTs and multiple DST fields

[kornel]

[Pre-RFC] Thin pointers with inlined metadata

Previous discussions: Idea: `Thin` wrapper of DSTs Summary This RFC adds Thin<T> that wraps T's metadata inline, which makes Thin<T> thin even if T is !Sized. Motivation Pointers of dynamically sized types (DSTs) are fat pointers, and they are not FFI-compatible, which obstables some common types like &str, &[T], and &dyn Trait from being passed across the FFI boundaries. 1. Passing pointers of DSTs accross FFI-boundary is hard Currently, it's difficult to using DSTs in FFI-compatible functi…