Don't be clever about representing non-CoW images (#4691)

This commit fixes a build warning on Rust 1.63 when the `memory-init-cow`
feature is disabled in the `wasmtime-runtime` crate. Some "tricks" were
used prior to have the `MemoryImage` type be an empty `enum {}` but that
wreaks havoc with warnings so this commit instead just makes it a unit
struct and makes all methods panic (as they shouldn't be hit anyway).

crates/runtime/src/lib.rs

@@ -19,7 +19,6 @@
         clippy::use_self
     )
 )]
-#![cfg_attr(not(memory_init_cow), allow(unused_variables, unreachable_code))]
 
 use anyhow::Error;
 use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};