Add aspectRatio constraint helper

[filipe-lemos]

Checklist

• I've rebased my changes on top of master
• I've built and run the project to see all new and existing tests pass
• I've followed the Mindera swift style guide
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

Some layouts are more easily achieved by defining a specific aspect
ratio for a view which, when combined with constraints to establish the
width or height of the view, allows Auto Layout to infer the size in
the other dimension.

Description

Add aspectRatio constraint helper through a new AspectRatioConstrainableProxy. This allows the creation of relative constraints between the width and height dimensions of a view.

[codecov]

Codecov Report

Merging #254 (b0df56b) into master (e1699ce) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #254      +/-   ##
==========================================
+ Coverage   95.04%   95.05%   +0.01%     
==========================================
  Files         100      100              
  Lines        3367     3376       +9     
==========================================
+ Hits         3200     3209       +9     
  Misses        167      167              

Impacted Files	Coverage Δ
Sources/AutoLayout/ConstrainableProxy.swift	98.10% <100.00%> (+0.04%)	⬆️

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[p4checo]

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