Suggestion: Use PluralRules instead of Count

[younies]

Summary

Currently, the enum Count is being used in multiple places around icu4x, such as units, currency, decimal ... etc.

• I suggest implement in PluralRules place an enum called l PluralRuleCount, which would contain all the CLDR counts:

pub enum PluralRulesCount {

  Zero,
  One,
  Two,
... etc.
}

• All data providers should adopt this standardized PluralRuleCount.
• If users need to use extra counts or variations, they should extend the existing PluralRuleCount.
  • Example:

     pub enum CompactDecimalCount {
          PluralRules(PluralRulesCount)
           Explicit1,
           Explicit0,
     }

[sffc]

The standard enum already exists: icu_plurals::PluralCategory

I like the idea of component-specific enums extending the standard one. Alternatively, I would be okay with icu_plurals exporting an enum with the explicit rules.

[younies]

In this case, we can use icu_plurals::PluralCategory.

But we need to address two things:

1. When the user extends the enum, there should be a function that handles, let's say, the first 4 bits for verifying and converting from and to the unaligned version of the PluralCategory. Then, the user can manipulate the remaining 4 bits as needed.
2. Ideally, it would be good if make_ule could handle this without a need of intervetion.

[younies]

Example of an extended enume: #5375

and a problem that need to be solved:
#5375 (comment)

[sffc]

• @Manishearth It's okay and expected for a manual ULE impl. make_ule won't work well here because it would need to share data with the child enum. Also, please remove the repr(u8).
• @echeran I noticed that this issue is talking about an enum to represent the explicit number matching, but there's an active discussion Match numbers numerically message-format-wg#842 with a proposal that the matching is not specific to integers, let alone to an enumerated list of integers.
• @sffc Currentlly, CLDR defines currency patterns for =0 and =1. When CLDR supports currency matching something like =73, then we can update our enum accordingly. But we are only supporting what CLDR specifies, and we don't need to design a general solution prematurely.
• @younies We could extend in the plurals crate for what is needed.
• @sffc One advantage of redefining the explicit enum in each crate is that each crate might need a different set of explicit values. An advantage of defining the enums in a common plae is that I'm not sure if it ever happens that different crates need different enums. It's always =0 and =1.
• @younies This plural implementation needs to have all categories that .. provides.
• @sffc Overall though I think it probably makes sense to have a single icu_plurals::PluralCategoryExtended.
• @Manishearth generally in favor of per-crate enums until we need something else. provider types are unstable so we can make such changes later. though it is nice to reduce unsafe
• @robertbastian The PluralRules type could return PluralCategoryExtended as well.
• @sffc It's good to return PluralRules becuase PluralCategoryExtended is a very ICU4X-specific type. I'm in favor of having both types and another function that returns the new PluralCategoryExtended type.
• @robertbastian If we had MessageFormat 2 (MF2) in the future, what would we do?
• @sffc This is not for MF2, this is a data provider concern for ICU4X.

Proposal:

• Add icu_plurals::PluralCategoryExtended which is PluralCategory but with Explicit0, Explicit1, and is non-exhaustive so more explicits can be added
• The type uses 4 bits.
• Add a function PluralRules::extended_category_for which returns PluralCategoryExtended

LGTM: @sffc @Manishearth @younies @echeran