Add prime numbers example

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b69f7ba) 98.37% compared to head (7ed8c6b) 98.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #56 +/- ## ======================================= Coverage 98.37% 98.38% ======================================= Files 63 63 Lines 2280 2287 +7 ======================================= + Hits 2243 2250 +7 Misses 37 37 ``` [see 2 files with indirect coverage changes](https://codecov.io/gh/tcbrindle/flux/pull/56/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tristan+Brindle)

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[tcbrindle]

This is a great little example of how to use flux::generator, thank you!