cdump: handle array sizes and comments better.

We can now parse array sizes properly: comments and extra [] pairs
will no longer confuse us.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
cdump: add CDUMP() support.

This lets you annotate your headers with notes for cdump.

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