Implementation of string.strip_prefix and string.strip_suffix

[GiregL]

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Here's a better implementation (a little less messy than the old one) for the string.strip_prefix and string.strip_suffix functions.

It is based on erlang and javascript in order to take care of the emoji fusions.

This PR references issue #743 .

Unit tests with from_utf_codepoints are there to take care of the zero width joiners used in some emojis like 👨‍👩‍👧‍👦.

I hope this one's fine now !