✨ Add zero for functions

[blegat]

Required by jump-dev/SumOfSquares.jl#71

[codecov-io]

Codecov Report

Merging #634 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #634      +/-   ##
=========================================
+ Coverage    95.2%   95.2%   +<.01%     
=========================================
  Files          48      48              
  Lines        5128    5130       +2     
=========================================
+ Hits         4882    4884       +2     
  Misses        246     246

Impacted Files	Coverage Δ
src/Utilities/functions.jl	97.17% <100%> (+0.01%)	⬆️

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[mlubin]

I should have taken a look at this earlier. We should not be using Base.zero() for mutable types (jump-dev/JuMP.jl#1151). Consider creating a new function instead of extending Base.zero.

[blegat]

Sorry for merging without review, I needed the changes of this PR and #610 at the same time.