Add numerator, denominator, terms and factors convenience functions

[hersle]

I think it is convenient when fiddling around with expressions to easily get numerators/denominators.

These functions are defined in Base, so I think it would be natural to extend them to Symbolics.

[hersle]

For some "completeness", I also added factors() and terms() to make it more intuitive/convenient to fiddle with arithmetic expression in general.

Do you think they make sense? If not I will revert.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 8.83%. Comparing base (eced900) to head (9a9e0bb).
Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
src/utils.jl	0.00%	20 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1445      +/-   ##
=========================================
- Coverage    8.88%   8.83%   -0.05%     
=========================================
  Files          51      51              
  Lines        4876    4903      +27     
=========================================
  Hits          433     433              
- Misses       4443    4470      +27     

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