This fixes an issue where if a store-incompatible value is returned from
a host-defined function then that value is leaked. Practically this
means that it's possible to accidentally leak `Func` values, but a
simple insertion of a `drop` does the trick!
`funcref`s are implemented as `NonNull<VMCallerCheckedAnyfunc>`.
This should be more efficient than using a `VMExternRef` that points at a
`VMCallerCheckedAnyfunc` because it gets rid of an indirection, dynamic
allocation, and some reference counting.
Note that the null function reference is *NOT* a null pointer; it is a
`VMCallerCheckedAnyfunc` that has a null `func_ptr` member.
Part of #929
