Compilation error with --threads:on
```nim
import jester
```
```
nim c --threads:on test.nim
```
```
.nimble/pkgs/jester-#head/jester.nim(237, 42) template/generic instantiation from here
Projects/nim/lib/pure/asyncmacro.nim(31, 8) Error: 'cb' is not GC-safe as it accesses 'nameIterVar' which is a global using GC'ed memory
```

Seems to be the same as #108, just sounds a bit more like show stopper :)