Function parse_base_sort

fn parse_base_sort(cx: &mut ParseCtxt<'_>) -> ParseResult<BaseSort>

⟨base_sort⟩ := bitvec < ⟨u32⟩ >
             | ⟨sort_path⟩ < ⟨base_sort⟩,* >
             | < ⟨ty⟩ as ⟨path⟩ > :: ⟨segment⟩
⟨sort_path⟩ := ⟨ident⟩ ⟨ :: ⟨ident⟩ ⟩* < (⟨base_sort⟩,*) >