Refactor shared memory internals, expose embedder methods (#5311)

This commit refactors the internals of `wasmtime_runtime::SharedMemory`
a bit to expose the necessary functions to invoke from the
`wasmtime::SharedMemory` layer. Notably some items are moved out of the
`RwLock` from prior, such as the type and the `VMMemoryDefinition`.
Additionally the organization around the `atomic_*` methods has been
redone to ensure that the `wasmtime`-layer abstraction has a single
method to call into which everything else uses as well.

crates/runtime/src/instance.rs

@@ -966,8 +966,8 @@ impl Instance {
                 let def_ptr = self.memories[defined_memory_index]
                     .as_shared_memory()
                     .unwrap()
-                    .vmmemory_ptr_mut();
-                ptr::write(ptr, def_ptr);
+                    .vmmemory_ptr();
+                ptr::write(ptr, def_ptr.cast_mut());
             } else {
                 ptr::write(owned_ptr, self.memories[defined_memory_index].vmmemory());
                 ptr::write(ptr, owned_ptr);