Fix sets usage

```
.../nimly/src/nimly/parsetypes.nim(165, 12) Warning: Deprecated since v0.20; sets are initialized by default; isValid is deprecated [Deprecated]
.../nimly/src/nimly/parsetypes.nim(166, 16) Warning: Deprecated since v0.20; sets are initialized by default; isValid is deprecated [Deprecated]
```