bug#51292: 27.2; Reversing strings with unicode combining characters

[Howard Melman]

Reversing a string fails to account for unicode combining characters

    (reverse "nai\u0308ve")
    "ev̈ian"

Note the diaeresis is now on the v and not the i.  s-reverse gets it right:

    (s-reverse "nai\u0308ve")
    "evïan"

I tried on both:

GNU Emacs 27.2 (build 1, x86_64-apple-darwin18.7.0, Carbon Version 158 AppKit 
1671.6) of 2021-03-27
GNU Emacs 27.1 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60 Version 
10.14.6 (Build 18G95)) of 2020-08-12

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Howard