Popog

Support changing the Command program

1

Since `Command` mostly adheres to a builder pattern, and `spawn` can be called multiple times, it would be useful to change the program. Motivating example: a user wants to fall...

Feature request: Retain Iterator

3

Have `retain` return a iterator over the extracted values, similar to `Drain` in the standard library, or my [PreRFC](https://internals.rust-lang.org/t/pre-rfc-retain-iterators-api-for-hashset-hashmap-vec-vecdeque/4164/4). It seems like a relatively simple change, so if you're interested...

Changed to Sync pool model

3

Changed threadpools to be Sync, which required the addition of a non-sync manager type in between Pool and Scoped. Added Sentinels to respawn threads (make respawn optional in the future?)....