wasmtime: Make StoreContextMut accessible in epoch deadline callback (#6075)

This commit changes the signature of the `Store::epoch_deadline_callback` to
take in `StoreContextMut` instead of a mutable reference to the store's data.

This is useful in cases in which the callback definition needs access to the
Store to be able to use other methods that take in `AsContext`/`AsContextMut`,
like for example `WasmtimeBacktrace::capture`

tests/all/epoch_interruption.rs

@@ -32,7 +32,7 @@ fn make_env<T>(engine: &Engine) -> Linker<T> {
 
 enum InterruptMode {
     Trap,
-    Callback(fn(&mut usize) -> Result<u64>),
+    Callback(fn(StoreContextMut<usize>) -> Result<u64>),
     Yield(u64),
 }
 
@@ -334,7 +334,8 @@ async fn epoch_callback_continue() {
                 (func $subfunc))
             ",
             1,
-            InterruptMode::Callback(|s| {
+            InterruptMode::Callback(|mut cx| {
+                let s = cx.data_mut();
                 *s += 1;
                 Ok(1)
             }),