1) Require "" around input
2) Make them optional around output: if not there, loose match whitespace
3) Handle \n in output.
4) Document that "Given xxx" is optional.
5) Reject any non-matching comment lines starting with "given" or "outputs"
6) Fix missed test in ccan/cast

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>