Make RBNF fall back to root

[c960657]

The RBNF for individual locales do not fall back to root. This is wrong. In particular, many locales rely on digits-ordinal being inherited from root.

The format currently exported by Rbnf is not suitable for merging. This PR suggests a different format. I do not have strong opinions on the specific format.

I am not sure about the intention behind Rbnf. I assume it is to avoid duplicating NumberingSystemRules in every locale. My patch is based on this assumption.

The output in xx/rbnf.yml looks like this:

---
da:
  rbnf:
    grouping:
      ordinal_rules:
        digits-ordinal:
          rules:
            "-x":
              rule: "−>>;"
            '0':
              rule: "=#,##0=.;"
      spellout_rules:
        and-small:
          access: private
          rules:
            '0':
              rule: og =%spellout-cardinal-common=;
            '100':
              rule: "=%spellout-cardinal-common=;"

The output in rbnf_root.yml looks like this:

---
rbnf:
  grouping:
    numbering_system_rules:
      cyrillic-lower-post:
        rules:
          '0':
            rule: ҃;
          '1':
            rule: "=%cyrillic-lower=;"
        access: private
      armenian-lower:
        rules:
          "-x":
            rule: "−>>;"
          x.x:
            rule: "=#,##0.00=;"
          '0':
            rule: 0;
          '1':
            rule: ա;