Support byte-based length fields in ref types

[KingMob]

To support AWS's event-stream format, I need length-prefixed specs. Unfortunately, string* is predefined to use int32, so I reached for ref-string. That handles the header value case, which is in16-based, but not the header name, which is byte-prefixed.

If you like, I'll submit a PR for it, just let me know.