Correct addition and subtraction operators for Point

[bash]

Resolves #23

[codecov]

Codecov Report

Merging #35 into master will increase coverage by 0.47%.
The diff coverage is 95.34%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
+ Coverage   83.18%   83.65%   +0.47%     
==========================================
  Files           7        7              
  Lines        1225     1236      +11     
  Branches      191      192       +1     
==========================================
+ Hits         1019     1034      +15     
+ Misses         27       22       -5     
- Partials      179      180       +1

Impacted Files	Coverage Δ
src/vector.rs	82.08% <100%> (+0.19%)	⬆️
src/polygon.rs	89.25% <93.33%> (-0.1%)	⬇️
src/point.rs	84.61% <95.65%> (+4.8%)	⬆️

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