fix stack alignement

- adjust align() in RawBlockTraits impl for Stack
- ensure ptr is always aligned in RawBlock::allock

src/machine/stack.rs

@@ -15,7 +15,9 @@ impl RawBlockTraits for Stack {
 
     #[inline]
     fn align() -> usize {
-        mem::align_of::<HeapCellValue>()
+        mem::align_of::<OrFrame>()
+            .max(mem::align_of::<AndFrame>())
+            .max(mem::align_of::<HeapCellValue>())
     }
 }
 
@@ -281,7 +283,6 @@ mod tests {
     use crate::machine::mock_wam::*;
 
     #[test]
-    #[cfg_attr(miri, ignore = "blocked on stack.rs UB")]
     fn stack_tests() {
         let mut wam = MockWAM::new();