Project Euler: First Ten solutions

[ddaniel27]

Hey, I tried to open a PR for Project Euler before (#721) but was rejected because very trivial solution. I agree with that decision, so I'm creating this new PR with the first ten solutions for Project Euler.

Let me know if you have any doubt or concern about this PR :)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.03846% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.70%. Comparing base (4263dac) to head (fb570ff).

Files with missing lines	Patch %	Lines
project_euler/problem_9/problem9.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
+ Coverage   87.53%   87.70%   +0.16%     
==========================================
  Files         212      222      +10     
  Lines        7110     7214     +104     
==========================================
+ Hits         6224     6327     +103     
- Misses        741      742       +1     
  Partials      145      145              

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