- Remove second optional parameter to natural_sort_key(), and all fraction
handling.

- Rename natsort_num2str() to natsort_encode_length() to show the intention
that it encodes string *lengths*, and not encode whitespaces and what not.

Handles lengths for which log10(len) >= 10,  even if they do not happen for
MariaDB Strings (where length is limited by 32bit, and log10(len) is <= 9)

- Do not let natural sort key grow past max_packet_length.


- Split Item_func_natural_sort_key::val_str() further and add
natsort_encode_numeric_string(), which contains comment on how
whitespaces are handled.

- Simplify, and speedup to_natsort_key() in common case, by removing
handling of weird charsets utf16/32, that encode numbers in several bytes.
In rare cases utf16/32 is used, we'll convert to utf8 prior to
creating keys, and back to original charset afterwards.