We use the fourth value (size) to determine the type, unless the fifth
value is suppled. That's silly: allow the fourth value to be a typename,
since that's the only reason we care about the size at all!
Unfortunately there are places in the spec where we use a raw fieldname
without '*1' for a length, so we have to distingish this from the
typename case.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
