✨ Add iszero

[blegat]

Fallbacks to zero(f) by default which is not implemented (only implemented for its type) and is disputed #636.
Required by jump-dev/SumOfSquares.jl#71

[codecov-io]

Codecov Report

Merging #643 into master will decrease coverage by 0.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #643      +/-   ##
==========================================
- Coverage   95.29%   94.91%   -0.38%     
==========================================
  Files          49       49              
  Lines        5272     5431     +159     
==========================================
+ Hits         5024     5155     +131     
- Misses        248      276      +28

Impacted Files	Coverage Δ
src/Utilities/functions.jl	96.71% <100%> (-0.89%)	⬇️
src/Utilities/parser.jl	86.71% <0%> (-10.94%)	⬇️
src/functions.jl	96.49% <0%> (-3.51%)	⬇️
src/Bridges/quadtosocbridge.jl	90.9% <0%> (-1.52%)	⬇️
src/Utilities/results.jl	87.64% <0%> (-1.13%)	⬇️
src/Test/modellike.jl	97.85% <0%> (-0.72%)	⬇️
src/Bridges/slackbridge.jl	100% <0%> (ø)	⬆️
src/Bridges/soctopsdbridge.jl	100% <0%> (ø)	⬆️
src/Bridges/lazybridgeoptimizer.jl	100% <0%> (+2.63%)	⬆️

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[mlubin]

Missing a test? The test should cover that iszero(f) is true iff f == zero(f) as the Julia documentation suggests.