Add canonicalize/canonicalize!

[tkoolen]

Used to re-express Parametron.Functions types in a standard form. Useful for easy comparison, among other things.

CC: @timholy. Decided against overloading == and hash, and remembered that @rdeits and I had implemented something similar for MathOptInterface functions.

[codecov]

Codecov Report

Merging #86 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   97.76%   97.86%   +0.09%     
==========================================
  Files           8        9       +1     
  Lines         493      515      +22     
==========================================
+ Hits          482      504      +22     
  Misses         11       11

Impacted Files	Coverage Δ
src/functions.jl	99.24% <100%> (+0.05%)	⬆️
src/util.jl	100% <100%> (ø)
src/model.jl	96.1% <0%> (-0.05%)	⬇️

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