Implement copy_if_while and copy_if_until

[jgopel]

Problem:

• There is no way to signal that a copy should proceed, selecting elements by a predicate until some condition is met. This is useful for patterns along the lines of "copy selected elements until there are n total elements in the output".

Solution:

• Introduce copy_if_while() and copy_if_until().

[jgopel]

@tzlaine Thanks for the feedback Zach - I've added documentation using the existing documentation for copy_while and copy_until as a template. Please let me know if there's anything that I can do to improve that 🙂

The documentation that I have is certainly not as thorough as your documentation is on #47 or on #99. I viewed this as a tradeoff between consistency and thoroughness. In my experience, consistency in documentation is very important, so I decided to prioritize that over thoroughness. I could have had both consistency and thoroughness by updating the existing documentation to be more thorough, but that felt out of scope for this PR. If that's something that would be a welcome addition, I would be more than happy to address that in another PR!

[tzlaine]

Docs LGTM. Marshall gets the final say, of course.

[jgopel]

This looks really good to me; just a couple bits of doxygen copy-pasta.

Sorry this took so long; my hand is healing very slowly

Thanks for the feedback! I'm sorry to hear your hand is healing slowly 😞 No rush on responding to the one comment that I had - if you need to wait out the hand that's totally fine, I'll be here and be motivated when you're ready!