feat request: sync analog of asyncio.gather

# Feature request

Future [containers](https://returns.readthedocs.io/en/latest/pages/future.html#future-container) are cool, but is not possible to concurrently execute them from sync context.


Like this
```
@future_safe
async def f()->Any:
  ...
@future_safe
async def g()->Any:
  ...

results = sync.gather(*[f(),g()])

```


