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

crates/api/src/trampoline/table.rs

@@ -1,12 +1,12 @@
 use super::create_handle::create_handle;
+use crate::trampoline::StoreInstanceHandle;
 use crate::Store;
 use crate::{TableType, ValType};
 use anyhow::{bail, Result};
 use wasmtime_environ::entity::PrimaryMap;
 use wasmtime_environ::{wasm, EntityIndex, Module};
-use wasmtime_runtime::InstanceHandle;
 
-pub fn create_handle_with_table(store: &Store, table: &TableType) -> Result<InstanceHandle> {
+pub fn create_handle_with_table(store: &Store, table: &TableType) -> Result<StoreInstanceHandle> {
     let mut module = Module::new();
 
     let table = wasm::Table {