Specialized planar convex hull

[blegat]

The 3D decomposition had a computation of the convex hull of the 2D facets that also supports rays and lines for a while, this PR extracts it as a separate planar_hull that is now called in removevredundancy! for 2D polyhedra.

[coveralls]

Coverage increased (+0.3%) to 91.783% when pulling 3ea7eb8 on bl/planar_convex into e03b49c on master.

[codecov]

Codecov Report

Merging #223 into master will increase coverage by 0.06%.
The diff coverage is 79.56%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   90.72%   90.79%   +0.06%     
==========================================
  Files          36       37       +1     
  Lines        2157     2215      +58     
==========================================
+ Hits         1957     2011      +54     
- Misses        200      204       +4     

Impacted Files	Coverage Δ
src/Polyhedra.jl	84.00% <ø> (ø)
src/recipe.jl	96.29% <ø> (+2.96%)	⬆️
src/decompose.jl	71.55% <37.77%> (-8.76%)	⬇️
src/planar.jl	100.00% <100.00%> (ø)
src/redundancy.jl	95.48% <100.00%> (+0.06%)	⬆️
src/elements.jl	84.40% <0.00%> (+0.53%)	⬆️

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