Ordered R list with optional element names (R list() semantics).
Each element is a {name, value} tuple where name is a String.t() for named
elements or nil for unnamed elements (R unnamed elements and empty-string names
both become nil). Order is preserved.
Construct directly: %Rx.RList{items: [{nil, 1.0}, {"b", 2.0}]}.
Access is not implemented; pattern-match on .items instead.