UTF-8 flag cleanup, and str_repeat

[nielsdos]

This abstracts away, and cleans up, the flag handling for properties of
strings that hold when concatenating two strings if they both hold that
property. (These macros also work with simply copies of strings because
a copy of a string can be considered a concatenation with the empty
string.) This gets rid of some branches and some repetitive code, and
leaves room for adding more flags like these in the future.

This also adds the UTF-8 flag for str_repeat (if the input is UTF-8).

cc @Girgias @alexdowad (because you both worked on this before)

[alexdowad]

Nice!

I would just suggest adding a code comment to ZSTR_COPYABLE_CONCAT_PROPERTIES to explain the idea behind this macro. (If I was reading this code without context, I would find it mysterious at first.)

[nielsdos]

Thanks, I added some comments.

[Girgias]

Oh that's a smarter way of doing it, I always forget that one can achieve this by using a bitwise AND.