Update: Restructure parts of f_utf project, and use f_utf_t, and add 'u' to hexdigits...

Update: Restructure parts of f_utf project, and use f_utf_t, and add 'u' to hexdigits in common.h.

Move the relevant functions into utf/convert.h, utf/is.h, and utf/is_character.h.

Implement f_utf_t as a type of uint32_t.
This should allow more customizability on that type if need be.
It also provides a more explicit context.

Add 'u' to designate the hexdigits are unsigned (This likely more specifically means it is an unsigned int).
This might be removed if it becomes a problem with architectures where "u" represents 16-bits or less.
Due to the size of the work, I am avoiding making this change in all of the Unicode comparison functions.
There are quite a lot of hexdigits in use.