Merge #1753

1753: Use a union for VMContext for functions r=MarkMcCaskey a=MarkMcCaskey

This makes the code more self documenting and correctly uses `unsafe`
to communicate that it's the caller's responsibility to ensure that the
code paths that can lead to the access can only write the value they
expect to be there.

spun off of #1739


- [x] There's still some inconsistent use of `vmctx` and `extra_data` might be good to unify this more.
- [x] Improve docs on the new type


Co-authored-by: Mark McCaskey <mark@wasmer.io>

lib/vm/src/lib.rs

@@ -49,9 +49,9 @@ pub use crate::table::{LinearTable, Table, TableStyle};
 pub use crate::trap::*;
 pub use crate::vmcontext::{
     VMBuiltinFunctionIndex, VMCallerCheckedAnyfunc, VMContext, VMDynamicFunctionContext,
-    VMFunctionBody, VMFunctionImport, VMFunctionKind, VMGlobalDefinition, VMGlobalImport,
-    VMMemoryDefinition, VMMemoryImport, VMSharedSignatureIndex, VMTableDefinition, VMTableImport,
-    VMTrampoline,
+    VMFunctionBody, VMFunctionEnvironment, VMFunctionImport, VMFunctionKind, VMGlobalDefinition,
+    VMGlobalImport, VMMemoryDefinition, VMMemoryImport, VMSharedSignatureIndex, VMTableDefinition,
+    VMTableImport, VMTrampoline,
 };
 pub use crate::vmoffsets::{TargetSharedSignatureIndex, VMOffsets};