deal with seq[T] casting to ptr SeqHeader with --gc:arc
```
type
  SeqHeader = object
    length, reserved: int

proc isLiteral*(s: string): bool {.inline.} =
  (cast[ptr SeqHeader](s.cstring).reserved and (1 shl (sizeof(int) * 8 - 2))) != 0

proc isLiteral*[T](s: seq[T]): bool {.inline.} =
  (cast[ptr SeqHeader](s).reserved and (1 shl (sizeof(int) * 8 - 2))) != 0
```
How to deal with seq[T] casting to ptr SeqHeader with --gc:arc