update bind:this references when setting to null

[Rich-Harris]

This fixes #2034 by using binding_callbacks when setting the foo in bind:this={foo} to null as well as when setting it to an element reference. Otherwise, the invalidation has no effect.

We still need to null out references before setting new ones, in order to avoid glitches when lists are reordered. This is done by unshifting to binding_callbacks the functions that need to run later, as those functions are run in reverse order.