Resolved options for ECMA-402

[sffc]

Most ECMA-402 components have a resolvedOptions() function.

I think there are 3 buckets of options in resolvedOptions(). Here is how we could address them:

• Locale
  • Available via DataProvider / DryDataProvider
  • Can be handled in wrapper
• Options that are passed in and never mutated
  • Do not need to record these in the low-level data model
  • Can be handled in wrapper
  • Examples:
    • plural rule options are all from the type construction or the input
    • list formatter is all from type construction
    • datetime DateStyle/TimeStyle is all from type construction
• Options that are computedN
  • These are what we actually need getters for
  • Examples:
    • decimal: numbering system
    • collator: stuff (already done)
    • datetime: calendar, hourCycle, numberingSystem

When I say "wrapper", I mean an ECMA-402 wrapper that sits in a standalone crate, which could be @filmil's crate icu4x_ecma402. We don't want to bloat the core types by the needs of ECMA-402, but we do want to make sure that all ECMA-402 requirements are achievable.

CC @jedel1043 @zbraniecki