807: Implement Send for Instance r=MarkMcCaskey a=MarkMcCaskey
# Review
- [x] Create a short description of the the change in the CHANGELOG.md file
Resolves#748
WIP
## List of changes
### Commit 1
- `Global`s use Arc instead of RC
- Export `Context` and `FuncPointer` manually implement Send
- `ImportObject` uses `Arc<Mutex<HashMap<...>>>` Instead of `Rc<RefCell<HashMap<...>>>`; removed `get_namespace` method in favor of continuation style to deal with locking the Mutex
- `Func` manually implements `Send` (TODO: this change needs to be checked in depth)
### Commit 2
- `unsafe impl Send for Export {}` (temporary to allow Memory to be not Send)
- RefCell -> Mutex in Global and Table
- Rc -> Arc in Table
- Namespace's `IsExport`s must be `Send` (Done to avoid touching much more of the code (i.e. `trait IsExport: Send` requires a lot -- maybe this is what we should do though)
- Make `Host` and `Wasm` `Func`s Send (manual implementation)
- Manual implementation for `LocalTable` and `AnyFunc`
### Commit 3
- rm placeholder `unsafe impl Send for Export {}`
- Manual implementation for `LocalBacking` and `ImportBacking` (both seemed to be not Send due to direct ownership of mutable pointers in their containers)
- ImportObject's state creator Fn trait object is now ` + Send + Sync + 'static` (required because it's in an Arc)
- Manually implement Send for `InstanceInner` because it holds a raw pointer, `LocalBacking` and `ImportBacking` are marked Send separately
- Memory: All Rc -> Arc (including unshared memory); All RefCell -> Mutex (including unshared memory)
- Manual implementation of Send and Sync on `UnsharedMemoryInternal`
- Manual implementation of Send and Sync on `SharedMemoryInternal`
- Change `runtime-core::unix::memory::Memory.fd` from `Option<Rc<Rawfd>>` to `Option<Arc<Rawfd>>` (not strictly required for this change because Memory has manual implementations of Send and Sync, but Arc seems more correct here and there's no comment justifying the use of Rc)
- Manual implementation of Send for `ImportedFunc`
Co-authored-by: Mark McCaskey <mark@wasmer.io>
Co-authored-by: Mark McCaskey <markmccaskey@users.noreply.github.com>
