Avoid copy between EmitResult and SmooshResult in m-c

[arai-a]

smoosh crate in m-c is currently copying jsparagus's EmitResult into SmooshResult, for the following reasons:

• binding needs #[repr(C)], and EmitResult doesn't have it
• we copy Vec<T> into CVec<T>
  • This can be solved if:
    • Vec<T> can be exposed to C, or
    • We add accessor functions that takes the reference
• To handle ownership and free it later
• (not yet in master) Option<T> needs some conversion, or type definition
• enum variant needs some conversion, or accessor
  • cbindgen can generate accessor