Update: Bring f_utf project in sync with f_string.

Update: Bring f_utf project in sync with f_string.

I have been explicitly avoiding making change to f_utf until I finished updating and successfully vetted the f_string project.
Now that the f_string has tests that pass, mass update the f_utf project.

This changes the functions and structure to be inline with how f_string works.
Many of the fixes are brought over.
I have observed that several of the bugs that were recently in f_string is also present in f_utf.
These bugs in f_utf should now be fixed as well.

This tests are brought over.
Only the tests for functions that are shared between f_string and f_utf are implemented.
And then those tests currently cannot be compiled.
The reason is that normal quoted strings cannot be used with the special UTF dynamic strings.
I simply did not have the time to get to this to finish writing these tests for f_utf.

There are some parts of f_string that make no sense in f_utf, such as f_string_range_t and f_string_quantity_t (both of which can be used on f_string and f_utf strings).
These have been omitted when copying the relevant f_string into f_utf code.