Progress: Continue mass converting to f_string_static_t and use const more.

Progress: Continue mass converting to f_string_static_t and use const more.

In particular this separates the fl_conversion functions into two forms:
1) That accepts only the f_string_static_t and uses dynamic.used as the length.
2) That accepts both f_string_static_t and f_string_range_t and uses the range to determine the length.

This makes the conversion functions more consistent with the rest of the project's practices.