shrink_to_fit some vectors to prevent interpreted code from passing the memory limits

src/memory.rs

@@ -179,12 +179,11 @@ pub fn reallocate(&mut self, ptr: Pointer, new_size: usize) -> EvalResult<'tcx,
             alloc.bytes.extend(iter::repeat(0).take(amount));
             alloc.undef_mask.grow(amount, false);
         } else if size > new_size {
-            // it's possible to cause miri to use arbitrary amounts of memory that aren't detectable
-            // through the memory_usage value, by allocating a lot and reallocating to zero
             self.memory_usage -= size - new_size;
             self.clear_relocations(ptr.offset(new_size as isize), size - new_size)?;
             let alloc = self.get_mut(ptr.alloc_id)?;
             alloc.bytes.truncate(new_size);
+            alloc.bytes.shrink_to_fit();
             alloc.undef_mask.truncate(new_size);
         }
 
@@ -676,6 +675,7 @@ fn grow(&mut self, amount: usize, new_state: bool) {
     fn truncate(&mut self, length: usize) {
         self.len = length;
         self.blocks.truncate(self.len / BLOCK_SIZE + 1);
+        self.blocks.shrink_to_fit();
     }
 }