Fix allocate_pstr randomly refusing to properly allocate memory

Fix allocate_pstr randomly refusing to properly allocate memory

This one was a toughie: it turns out that using `ptr::align_of()`` was
a bad idea, since the buffer in `Heap` itself is not aligned to
`Heap::heap_cell_alignment()`, so `ptr::align_of()` would sometimes
return lower values than expected.

That made for an heisenbug: if the alignment of the heap happened to be 4,
then the bug wouldn't trigger.