htable: add and integrate htable_check function.

Trying to debug what looked like an htable fail in my own code (but seems
it's not), I added htable_check a-la list_check et al.

This means we now depend on ccan/str (for stringify), which breaks the
_info example which also defined streq, and played nasty with const pointers.

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