0.4

• use include Seq so we're forward compatible with OCaml
• add relational operators: group_by, group_by_fold, join_by, join_by_fold
• add {flat_map,app}_interleave, to_iter, to_gen
• add head_exn/tail_exn
• fix: make product more fair by using interleave
• fix: handle exceptions in OSeq.memoize