Project Euler: First Twenty 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 :)

Update 1:
Added solutions from 11 to 15

Update 2:
Added solutions from 16 to 20. I'm gonna stop updating this PR while is getting merge. I don't want to make a large(r) PR.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.77483% with 98 lines in your changes missing coverage. Please review.

Project coverage is 87.48%. Comparing base (82b6e96) to head (1c1a101).

Files with missing lines	Patch %	Lines
project_euler/problem_18/tree.go	51.19%	81 Missing and 1 partial ⚠️
project_euler/problem_11/problem11.go	80.00%	4 Missing and 2 partials ⚠️
project_euler/problem_18/root.go	87.50%	4 Missing ⚠️
project_euler/problem_20/problem20.go	80.00%	2 Missing and 1 partial ⚠️
project_euler/problem_18/problem18.go	77.77%	1 Missing and 1 partial ⚠️
project_euler/problem_9/problem9.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
- Coverage   87.79%   87.48%   -0.31%     
==========================================
  Files         215      239      +24     
  Lines        7258     7862     +604     
==========================================
+ Hits         6372     6878     +506     
- Misses        741      834      +93     
- Partials      145      150       +5     

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