sd-id128: rename and export sd_id128_string_equal()

sd-id128: rename and export sd_id128_string_equal()

We find this function useful in our code, so no reason not to export it.
I changed the order of last two words in the name to match the arguments.
(With "equal_string" I expected sd_id128_t first, string second, but in
actual use, the second argument is usually a long constant so it's nice
to keep this order of arguments.)