CABI: Add a `resource.consume` method for owned handles

Motivation

At the moment, there is no way to convert an owned resource handle inside the resource-exporting component back into an owned value of the type that implements the resource. While the specifics of performing this conversion can be left to wit-bindgen (see, e.g., https://github.com/bytecodealliance/wit-bindgen/issues/641#issuecomment-1686866481), it requires a canonical ABI primitive to consume an owned resource handle.

Detail

The canonical ABI defines a new method, resource.consume, which might be implemented as follows:

### `canon resource.consume`

def canon_resource_consume(inst, rt, i):
  # similar to resource.drop, but only for owned handles
  h = inst.handles.remove(rt, i)
  trap_if(!h.own)
  assert(h.scope is None)
  trap_if(h.lend_count != 0)
  trap_if(inst is not rt.impl and not rt.impl.may_enter)

  # note: do *not* call h's destructor

  # similar to resource.rep
  return h.rep

In particular, resource.consume, which has a (handwavy) signature of fn(owned_handle: i32) -> isize:

• takes an owned handle
• deallocates the handle, like resource.drop
• does not call the resource's destructor, since this method may be used to reobtain ownership of the resource's implementing value
• returns the handle's representation, like resource.rep

Closing Notes

Thank you for considering this extension to the canonical ABI :)

Start by reading the canonical ABI behavior of resource.drop and resource.rep, then review the linked wit-bindgen discussion for the proposed conversion use case. Done means the canonical ABI specifies resource.consume's ownership, lending, destructor, trapping, and representation semantics clearly enough for wit-bindgen to rely on.