Change some VMContext pointers to () pointers (#4190)

* Change some `VMContext` pointers to `()` pointers

This commit is motivated by my work on the component model
implementation for imported functions. Currently all context pointers in
wasm are `*mut VMContext` but with the component model my plan is to
make some pointers instead along the lines of `*mut VMComponentContext`.
In doing this though one worry I have is breaking what has otherwise
been a core invariant of Wasmtime for quite some time, subtly
introducing bugs by accident.

To help assuage my worry I've opted here to erase knowledge of
`*mut VMContext` where possible. Instead where applicable a context
pointer is simply known as `*mut ()` and the embedder doesn't actually
know anything about this context beyond the value of the pointer. This
will help prevent Wasmtime from accidentally ever trying to interpret
this context pointer as an actual `VMContext` when it might instead be a
`VMComponentContext`.

Overall this was a pretty smooth transition. The main change here is
that the `VMTrampoline` (now sporting more docs) has its first argument
changed to `*mut ()`. The second argument, the caller context, is still
configured as `*mut VMContext` though because all functions are always
called from wasm still. Eventually for component-to-component calls I
think we'll probably "fake" the second argument as the same as the first
argument, losing track of the original caller, as an intentional way of
isolating components from each other.

Along the way there are a few host locations which do actually assume
that the first argument is indeed a `VMContext`. These are valid
assumptions that are upheld from a correct implementation, but I opted
to add a "magic" field to `VMContext` to assert this in debug mode. This
new "magic" field is inintialized during normal vmcontext initialization
and it's checked whenever a `VMContext` is reinterpreted as an
`Instance` (but only in debug mode). My hope here is to catch any future
accidental mistakes, if ever.

* Use a VMOpaqueContext wrapper

* Fix typos

crates/runtime/src/instance.rs

@@ -9,8 +9,8 @@ use crate::table::{Table, TableElement, TableElementType};
 use crate::traphandlers::Trap;
 use crate::vmcontext::{
     VMBuiltinFunctionsArray, VMCallerCheckedAnyfunc, VMContext, VMFunctionImport,
-    VMGlobalDefinition, VMGlobalImport, VMMemoryDefinition, VMMemoryImport, VMRuntimeLimits,
-    VMTableDefinition, VMTableImport,
+    VMGlobalDefinition, VMGlobalImport, VMMemoryDefinition, VMMemoryImport, VMOpaqueContext,
+    VMRuntimeLimits, VMTableDefinition, VMTableImport, VMCONTEXT_MAGIC,
 };
 use crate::{
     ExportFunction, ExportGlobal, ExportMemory, ExportTable, Imports, ModuleRuntimeInfo, Store,
@@ -488,7 +488,7 @@ impl Instance {
                 (self.runtime_info.image_base()
                     + self.runtime_info.function_info(def_index).start as usize)
                     as *mut _,
-                self.vmctx_ptr(),
+                VMOpaqueContext::from_vmcontext(self.vmctx_ptr()),
             )
         } else {
             let import = self.imported_function(index);
@@ -879,6 +879,8 @@ impl Instance {
     unsafe fn initialize_vmctx(&mut self, module: &Module, store: StorePtr, imports: Imports) {
         assert!(std::ptr::eq(module, self.module().as_ref()));
 
+        *self.vmctx_plus_offset(self.offsets.vmctx_magic()) = VMCONTEXT_MAGIC;
+
         if let Some(store) = store.as_raw() {
             *self.runtime_limits() = (*store).vmruntime_limits();
             *self.epoch_ptr() = (*store).epoch_ptr();