[Str] Add the ability to supply string encoding

[azjezz]

Add the ability to supply string encoding instead of detecting the encoding from the giving string.

falling back to the internal encoding, then 'UTF-8'.

Now Psl\Str can be complete replacement for mb_str* and str* functions 🎉

[coveralls]

Pull Request Test Coverage Report for Build 518

• 88 of 88 (100.0%) changed or added relevant lines in 38 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 512:	0.0%
Covered Lines:	1663
Relevant Lines:	1663

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💛 - Coveralls