Break out numbering systems from QLocale

A locale may use several numbering systems - e.g. Chinese uses different number symbols for days than for counting sheep; there may also be locales that use Latin digits for some purposes but some other numbering system for others - and they are things separate from locales in the upstream CLDR data. This shall be a prerequisite of some calendar implementations.

Doing this would also allow us to actually record the full range of digits, instead of only recording 0 and having to do kludges to work out the rest. We would also need to do this, as some numbering systems may not, historically, have ever used a 0 until one was added in relatively modern times, which may lead to several distinct numbering systems using the same zero.

This would replace the zero-entry (an offset into one of our string data tables, plus a length) in the QLocale data tables with a number-system ID – which would surely in fact make them smaller (by approximately 680 words = 5440 bytes), while replacing the zeros in our "single-character" table with a new table for the digit-arrays, plus the new table for number systems.
Since these last two would be fairly small (many of the locales we support share a small number of number systems) the change in data size shouldn't be a concern (and we might even make a saving).

CLDR also has diverse numbering systems for which, instead of just listing the digits, it provides an algorithmic implementation; we should at least design with the possibility of eventually including support for these, albeit the initial separation from QLocale does not need this.