Feature: add f_string_length_size max of (uint64_t - 4)

Feature: add f_string_length_size max of (uint64_t - 4)

The goal here is that any string processing must be able to add a given UTF-8 width (4-byte) and not oveflow.
Much of the code in this project will not check this as it should be done so at a higher level (performance reasons).
The ideal time is that when allocating some string, always allocate at max f_string_length_size.