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/wasi-common/wig/src/wasi.rs

@@ -201,8 +201,8 @@ pub fn add_wrappers_to_module(args: TokenStream) -> TokenStream {
                         #format_str,
                         #(#format_args),*
                     );
-                    let wasi_ctx = match get_wasi_ctx(&mut *ctx) {
-                        Ok(e) => e,
+                    let mut wasi_ctx = match get_wasi_ctx(&mut *ctx) {
+                        Ok(e) => e.borrow_mut(),
                         Err(e) => #handle_early_error,
                     };
                     let memory = match get_memory(&mut *caller_ctx) {
@@ -210,7 +210,7 @@ pub fn add_wrappers_to_module(args: TokenStream) -> TokenStream {
                         Err(e) => #handle_early_error,
                     };
                     hostcalls::#name_ident(
-                        wasi_ctx,
+                        &mut *wasi_ctx,
                         memory,
                         #(#hostcall_args),*
                     ) #cvt_ret