Alternative architecture for smartptrs

[planetis-m]

To accommodate the use-cases specified in #5 (comment), here is my proposed alternative:

https://github.com/planetis-m/dumpster/blob/master/expsmarts.nim#L18-L30

type
  SharedPtr*[T] = object
    ## Shared ownership reference counting pointer.
    deleter*: Deleter[T]
    val*: ptr Payload[T]

proc `=destroy`*[T](p: var SharedPtr[T]) =
  if p.val != nil:
    if p.val.counter.load(Consume) == 0:
      `=destroy`(p.val.value)
      if p.deleter != nil:
        p.deleter(p.val[])
    else:
      atomicDec(p.val.counter)

Can the deleter field be removed somehow?
Every field must be exported, so that libraries can provide their implementations like this:

https://github.com/planetis-m/dumpster/blob/master/pools.nim#L24

Opinions? @arnetheduck, @timotheecour, @Araq

[timotheecour]

I know @Araq suggested this repo, but why not use fusion for that? soon enough we'll end up with mutual dependencies (fusion needs threading or vice versa) or, more likely, we'll end up writing sub-optimal workarounds because of that

[Araq]

Here it can be developed and ripe before it becomes part of the stdlib. I don't see how mutual deps would arise.