ccan/structeq: make it safe when there's padding.

ccan/cppmagic FTW!

The only issue is that we can't tell if there's padding or they've missed
a member, so we add a padding bytes count, so they'll get an error if it
(for example) the structure adds a new member later.

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