Add fn for splitting borrow of memory & data in Ctx, use in WASI

lib/runtime-core/src/vm.rs

@@ -401,6 +401,21 @@ impl Ctx {
         }
     }
 
+    /// Splits the borrow, getting both Memory and a mutable reference to
+    /// the data type provided.
+    ///
+    /// This method is required to access both at the same time.
+    /// This is useful if the data stores information about locations in Wasm
+    /// memory that a hostcall needs to access.
+    ///
+    /// # Safety
+    ///
+    /// This function must be called with the same type, `T`, that the `data`
+    /// was set to.
+    pub unsafe fn memory_and_data_mut<T>(&mut self, mem_index: u32) -> (&Memory, &mut T) {
+        (self.memory(mem_index), &mut *(self.data as *mut T))
+    }
+
     /// Gives access to the emscripten symbol map, used for debugging
     pub unsafe fn borrow_symbol_map(&self) -> &Option<HashMap<u32, String>> {
         &(*self.module).info.em_symbol_map