It speeds up
```nim
proc foo =
let piece = cast[seq[char]](newSeqUninit[uint8](5220600386'i64))
foo()
```
Notes that `cast[ref](...)` is excluded because we need to keep the ref
alive if the parameter is something with pointer types (e.g.
`cast[ref](pointer)`or `cast[ref](makePointer(...))`)
---------
Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
4ef06a5cc5