Parallel task compilation fails for nim >= 2.0.0

Compiling the task parallelism example in the readme with any version of nim over 2 results in this compilation error:
```
/path/to/.nimble/pkgs/weave-0.4.10/weave/parallel_tasks.nim(171, 16) Error: type mismatch: got 'proc (param`gensym2: pointer){.nimcall.}' for 'async_fib' but expected 'proc (param: pointer){.nimcall, gcsafe.}'
  Pragma mismatch: got '{..}', but expected '{.gcsafe.}'.
```