Add unrolled_enumerate and unrolled_applyat

[dennisYatunin]

This PR adds 2 new unrolled functions, along with relevant tests and documentation:

• unrolled_enumerate, which is similar to enumerate (but also able to handle multiple iterators at the same time)
• unrolled_applyat, which applies a function to the item at a particular index of an iterator (or to the items at that index in multiple iterators)

The second function should be a simpler alternative to TuplesOfNTuples.jl.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (5c18833) to head (72bfe93).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           43        46    +3     
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+ Hits            43        46    +3     

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