adds div and simplifications for div, mod and rem

[rmsrosa]

I also added a bunch of tests, which highlight the simplifications added.

There are still some simplifications that can be included. I will work on those, but I think/hope this is a good start.

I haven't added anything to the documentation either. I can work on that later once I figure out how it is deployed.

Let me know if there is anything I should do differently. In particular regarding the way I added the rules to default_simplifier. I wanted to do minimal changes to that, but I suspect separating div/rem/mod from trig/exp simplifications might be best since they will certainly be less used.

[rmsrosa]

Ok, I made my last commit of the series. I added the missing simplifications and adjusted testing. It is good to go.

[rmsrosa]

Hmm, is there anything missing from my part? It still shows a change request but I believe I addressed that, already. I thought this was good to go. Let me know if I am missing something.

[YingboMa]

@shashi could you take a look?

[codecov-commenter]

Codecov Report

Merging #418 (d994a48) into master (9bbd946) will decrease coverage by 0.60%. The diff coverage is 48.14%.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
- Coverage   83.99%   83.39%   -0.61%     
==========================================
  Files          13       13              
  Lines        1387     1409      +22     
==========================================
+ Hits         1165     1175      +10     
- Misses        222      234      +12     

Impacted Files	Coverage Δ
src/methods.jl	83.63% <ø> (ø)
src/simplify_rules.jl	40.00% <22.22%> (-51.67%)	:arrow_down:
src/utils.jl	65.00% <100.00%> (+7.25%)	:arrow_up:
src/polyform.jl	93.72% <0.00%> (-0.42%)	:arrow_down:
src/ordering.jl	89.65% <0.00%> (+1.14%)	:arrow_up:

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[rmsrosa]

Ok, I can take a look at the codecov stuff. Maybe some rules are redundant or tests were not exhaustive.

But I don't know about the failed IntegrationTest/Symbolics.jl. Was that introduced by these changes or existed already?