Implement strstr, strcasestr. Fixes #713

[corona10]

Fixes: #713

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This change is 

[codecov]

Codecov Report

Merging #714 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #714      +/-   ##
==========================================
+ Coverage      82%   82.03%   +0.02%     
==========================================
  Files         164      164              
  Lines        9042     9056      +14     
==========================================
+ Hits         7415     7429      +14     
  Misses       1223     1223              
  Partials      404      404

Impacted Files	Coverage Δ
program/function_definition.go	100% <ø> (ø)	⬆️
noarch/string.go	100% <100%> (ø)	⬆️

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[elliotchance]

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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noarch/string.go, line 80 at r1 (raw file):

	}
	return str1[index : index+len(b)]
}

Nice! 👍

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