feat: implement a cfg macro for joining two closures

[dkales]

Description

This PR adds a cfg macro for the execution of two closures. The execution will happen in parallel using rayon::join if possible, and the macro can also be nested like normal rayon::join.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[Pratyush]

Thanks for the PR! There’s a CI failure, but looks good otherwise!

[dkales]

sry about that, copied it down there for formatting and forgot...