Macro impl_vec_try_from_sexp_list 

macro_rules! impl_vec_try_from_sexp_list {
    ($t:ty) => { ... };
}

Implement TryFromSexp for Vec<T> from R list (VECSXP).

Each element is converted via T::try_from_sexp. Crate-internal, like impl_option_try_from_sexp!: Vec<T> impls are orphans downstream.