fails to print bit field sets

https://nim-lang.org/docs/manual.html#set-type-bit-fields

```nim
import print

type
  MyFlag* {.size: sizeof(cint).} = enum
    mfA = 1
    mfB
    mfC
  Foo = object
    flags: set[MyFlag]
```
```
print-1.0.1/print.nim(164, 55) Error: type mismatch: got <set[MyFlag]>
```

