Reduce goroutine overhead by abstracting away the process of goroutine procurement.

[jordanhasgul]

Currently, fallible functions are executed in a new goroutine when (*errgroup.Group).Go is called. This creates some overhead when running a large number of fallible functions as we have to pay the price of creating a new goroutine each time.

Instead, we could abstract away the procurement of goroutines as we only care that our fallible function is executed in another goroutine - it does not necessarily need to be a new one. This way, we could allow the consumer to choose whether they would like to execute a fallible function in a new goroutine or have it executed by a single goroutine from a pool of existing goroutines.