Reel in unsafety around InstanceHandle (#856)

* Reel in unsafety around `InstanceHandle`

This commit is an attempt, or at least is targeted at being a start, at
reeling in the unsafety around the `InstanceHandle` type. Currently this
type represents a sort of moral `Rc<Instance>` but is a bit more
specialized since the underlying memory is allocated through mmap.

Additionally, though, `InstanceHandle` exposes a fundamental flaw in its
safety by safetly allowing mutable access so long as you have `&mut
InstanceHandle`. This type, however, is trivially created by simply
cloning a `InstanceHandle` to get an owned reference. This means that
`&mut InstanceHandle` does not actually provide any guarantees about
uniqueness, so there's no more safety than `&InstanceHandle` itself.

This commit removes all `&mut self` APIs from `InstanceHandle`,
additionally removing some where `&self` was `unsafe` and `&mut self`
was safe (since it was trivial to subvert this "safety"). In doing so
interior mutability patterns are now used much more extensively through
structures such as `Table` and `Memory`. Additionally a number of
methods were refactored to be a bit clearer and use helper functions
where possible.

This is a relatively large commit unfortunately, but it snowballed very
quickly into touching quite a few places. My hope though is that this
will prevent developers working on wasmtime internals as well as
developers still yet to migrate to the `wasmtime` crate from falling
into trivial unsafe traps by accidentally using `&mut` when they can't.
All existing users relying on `&mut` will need to migrate to some form
of interior mutability, such as using `RefCell` or `Cell`.

This commit also additionally marks `InstanceHandle::new` as an `unsafe`
function. The rationale for this is that the `&mut`-safety is only the
beginning for the safety of `InstanceHandle`. In general the wasmtime
internals are extremely unsafe and haven't been audited for appropriate
usage of `unsafe`. Until that's done it's hoped that we can warn users
with this `unsafe` constructor and otherwise push users to the
`wasmtime` crate which we know is safe.

* Fix windows build

* Wrap up mutable memory state in one structure

Rather than having separate fields

* Use `Cell::set`, not `Cell::replace`, where possible

* Add a helper function for offsets from VMContext

* Fix a typo from merging

* rustfmt

* Use try_from, not as

* Tweak style of some setters

crates/runtime/src/vmcontext.rs

@@ -605,16 +605,16 @@ impl VMContext {
     /// This is unsafe because it doesn't work on just any `VMContext`, it must
     /// be a `VMContext` allocated as part of an `Instance`.
     #[allow(clippy::cast_ptr_alignment)]
-    pub(crate) unsafe fn instance(&mut self) -> &mut Instance {
-        &mut *((self as *mut Self as *mut u8).offset(-Instance::vmctx_offset()) as *mut Instance)
+    pub(crate) unsafe fn instance(&self) -> &Instance {
+        &*((self as *const Self as *mut u8).offset(-Instance::vmctx_offset()) as *const Instance)
     }
 
-    /// Return a mutable reference to the host state associated with this `Instance`.
+    /// Return a reference to the host state associated with this `Instance`.
     ///
     /// # Safety
     /// This is unsafe because it doesn't work on just any `VMContext`, it must
     /// be a `VMContext` allocated as part of an `Instance`.
-    pub unsafe fn host_state(&mut self) -> &mut dyn Any {
+    pub unsafe fn host_state(&self) -> &dyn Any {
         self.instance().host_state()
     }
 }