Add Difference operator

[sharanry]

SciML/ModelingToolkit.jl#1088 (comment)

This PR aims to add the Difference operator to enable defining fixed discrete step state transitions.

@YingboMa
CC: @ChrisRackauckas

[sharanry]

I am unsure what kind of functionality needs to be offered by the Difference operator as part of Symbolics.
Going through the Differential operator, I noticed that it offers compositions, equality, expand_derivatives to name a few -- I am still trying to understand what would be the equivalent for the Difference operator. I have started off by implementing basic equality and hashing functionality.

[ChrisRackauckas]

For now we can keep it barebones and add features to it as it grows.

[codecov-commenter]

Codecov Report

Merging #293 (d65c4bf) into master (eeee436) will increase coverage by 0.16%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
+ Coverage   75.78%   75.95%   +0.16%     
==========================================
  Files          19       20       +1     
  Lines        1817     1834      +17     
==========================================
+ Hits         1377     1393      +16     
- Misses        440      441       +1     

Impacted Files	Coverage Δ
src/Symbolics.jl	100.00% <ø> (ø)
src/difference.jl	87.50% <87.50%> (ø)
src/utils.jl	71.05% <0.00%> (ø)
src/num.jl	64.28% <0.00%> (+0.64%)	⬆️
src/build_function.jl	69.06% <0.00%> (+0.79%)	⬆️

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