Error with strformat + asyncdispatch + const

### Example
```nim
import strformat, asyncdispatch

const x = ""

proc my_proc{.async.} =
  const _ = &"{x}"
```

### Current Output
```
...\lib\pure\strformat.nim(532, 19) Error: cannot evaluate at compile time: fmtRes_448056
```

### Additional Information

```
Nim Compiler Version 1.0.2 [Windows: amd64]
```
