Add WaitAndRecover method to WaitGroup

[craigpastro]

Resolves #29.

👋 My attempt at resolving #29. I wasn't sure if was better to reimplement Wait in terms of WaitSafe. I think leaving the code as it looks a bit cleaner, so I just left it.

I also wondered if it is better to say in the docs that WaitSafe will return the first panic it encounters and add a test for that. That is true, right?

Thank you!

[codecov-commenter]

Codecov Report

Merging #53 (83a01d6) into main (559008f) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   99.44%   99.45%           
=======================================
  Files          10       10           
  Lines         359      364    +5     
=======================================
+ Hits          357      362    +5     
  Misses          2        2           

Impacted Files	Coverage Δ
waitgroup.go	100.00% <100.00%> (ø)

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