Revamp memory management of InstanceHandle (#1624)

* Revamp memory management of `InstanceHandle`

This commit fixes a known but in Wasmtime where an instance could still
be used after it was freed. Unfortunately the fix here is a bit of a
hammer, but it's the best that we can do for now. The changes made in
this commit are:

* A `Store` now stores all `InstanceHandle` objects it ever creates.
  This keeps all instances alive unconditionally (along with all host
  functions and such) until the `Store` is itself dropped. Note that a
  `Store` is reference counted so basically everything has to be dropped
  to drop anything, there's no longer any partial deallocation of instances.

* The `InstanceHandle` type's own reference counting has been removed.
  This is largely redundant with what's already happening in `Store`, so
  there's no need to manage two reference counts.

* Each `InstanceHandle` no longer tracks its dependencies in terms of
  instance handles. This set was actually inaccurate due to dynamic
  updates to tables and such, so we needed to revamp it anyway.

* Initialization of an `InstanceHandle` is now deferred until after
  `InstanceHandle::new`. This allows storing the `InstanceHandle` before
  side-effectful initialization, such as copying element segments or
  running the start function, to ensure that regardless of the result of
  instantiation the underlying `InstanceHandle` is still available to
  persist in storage.

Overall this should fix a known possible way to safely segfault Wasmtime
today (yay!) and it should also fix some flaikness I've seen on CI.
Turns out one of the spec tests
(bulk-memory-operations/partial-init-table-segment.wast) exercises this
functionality and we were hitting sporating use-after-free, but only on
Windows.

* Shuffle some APIs around

* Comment weak cycle

tests/all/func.rs

@@ -39,6 +39,7 @@ fn dtor_runs() {
     Func::wrap(&store, move || {
         drop(&a);
     });
+    drop(store);
     assert_eq!(HITS.load(SeqCst), 1);
 }
 
@@ -63,7 +64,7 @@ fn dtor_delayed() -> Result<()> {
     let module = Module::new(&store, &wasm)?;
     let instance = Instance::new(&module, &[func.into()])?;
     assert_eq!(HITS.load(SeqCst), 0);
-    drop(instance);
+    drop((instance, module, store));
     assert_eq!(HITS.load(SeqCst), 1);
     Ok(())
 }