This is a primitive mark-and-sweep-style garbage detector. The core is
in common/ for later use by subdaemons, but for now it's just lightningd.
We initialize it before most other allocations.
We walk the tal tree to get all the pointers, then search the `ld`
object for those pointers, recursing down. Some specific helpers are
required for hashtables (which stash bits in the unused pointer bits,
so won't be found).
There's `notleak()` for annotating things that aren't leaks: things
like globals and timers, and other semi-transients.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
