val into_cstruct : t -> Cstruct.t -> (unit, error) Stdlib.resultinto_cstruct t buf writes a 14-byte ethernet header representing t.ethertype, t.src_mac, and t.dst_mac to buf at offset 0. Returns Ok () on success and Error error on failure. Currently, the only possibility for failure is a buf too small to contain the header; to avoid this, provide a buffer of size at least 14.