Consider adding a `free()` function

[oold]

Especially in the context of using utf8proc as a shared library, there might be cases where the user's malloc implementation differs from what the utf8proc library uses internally. It's generally considered a good practice to add a deallocation function for data allocated by a library. I'd suggest creating a new function called utf8proc_free().

Examples of such deallocation functions:

• https://docs.gtk.org/glib/func.free.html
• https://sqlite.org/capi3ref.html#sqlite3_free
• https://curl.se/libcurl/c/curl_free.html

Here is why using free() is a bad idea if utf8proc is a shared library (or just built against a different CRT):

• https://learn.microsoft.com/en-us/cpp/c-runtime-library/potential-errors-passing-crt-objects-across-dll-boundaries
• https://stackoverflow.com/a/13587381/19111529