Implement strings utility library instead of our own

[cliffordp]

https://laravel.com/docs/master/helpers --> https://packagist.org/packages/laravel/helpers requires PHP 7.1.3

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https://packagist.org/packages/symfony/string requires PHP 7.2.5

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https://github.com/PHLAK/Twine requires PHP 7.2

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https://github.com/phpfunct/funct seems to cover it all for both strings and arrays (re: #87) but isn't actively maintained.

Strings:

• contains
• starts with
• ends with
• truncate

One thing missing from this library is to add prefix while accounting for it possibly existing already (and same for suffix): phpfunct/funct#47

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https://packagist.org/packages/voku/stringy requires PHP 7.0, has all the string functions, and is actively maintained

So that's why we chose this one.