ZJIT: Replace std::mem::transmute with pointer casting

As the documentation puts it, transmute is "incredibly unsafe". We can
express what we need to with pointer casts in these closure FFI
situations, so let's use pointer casts.

Author: XrXr

zjit/src/cruby.rs

@@ -320,11 +320,12 @@ pub fn iseq_rest_param_idx(params: &IseqParameters) -> Option<i32> {
 pub fn for_each_iseq<F: FnMut(IseqPtr)>(mut callback: F) {
     unsafe extern "C" fn callback_wrapper(iseq: IseqPtr, data: *mut c_void) {
         // SAFETY: points to the local below
-        let callback: &mut &mut dyn FnMut(IseqPtr) -> bool = unsafe { std::mem::transmute(&mut *data) };
-        callback(iseq);
+        let callback: *mut *mut dyn FnMut(IseqPtr) -> bool = data.cast();
+        unsafe { (**callback)(iseq) };
     }
-    let mut data: &mut dyn FnMut(IseqPtr) = &mut callback;
-    unsafe { rb_jit_for_each_iseq(Some(callback_wrapper), (&mut data) as *mut _ as *mut c_void) };
+    let mut data: *mut dyn FnMut(IseqPtr) = &raw mut callback;
+    let data: *mut *mut dyn FnMut(IseqPtr) = &raw mut data;
+    unsafe { rb_jit_for_each_iseq(Some(callback_wrapper), data.cast()) };
 }
 
 /// Return a poison value to be set above the stack top to verify leafness.