Functions to implement the number system rule-based-number-format rules of CLDR.

These rules are defined only on the "root" locale and represent specialised number formatting.

The standard public API for RBNF is via the Cldr.Number.to_string/2 function.

The functions on this module are defined at compile time based upon the RBNF rules defined in the Unicode CLDR data repository. Available rules are identified by:

iex> Cldr.Print.Backend.Rbnf.NumberSystem.rule_sets("root")
[:tamil, :roman_upper, :roman_lower, :hebrew_item, :hebrew_0_99, :hebrew,
:greek_upper, :greek_lower, :georgian, :ethiopic_p1, :ethiopic,
:cyrillic_lower_1_10, :cyrillic_lower, :armenian_upper, :armenian_lower]

A rule can then be invoked on an available rule_set. For example

iex> Cldr.Print.Backend.Rbnf.NumberSystem.roman_upper(123, "root")
"CXXIII"

This call is equivalent to the call through the public API of:

iex> Cldr.Print.Backend.Number.to_string(123, format: :roman)
{:ok, "CXXIII"}