Add PluralRules Categories functionality

[nordzilla]

ICU4X currently has no functionality to return an enumeration of keywords that are supported by a PluralRules object for a given locale and Cardinal/Ordinal setting.

The equivalent call that we currently use in SpiderMonkey is uplrules_getKeywords()
https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/upluralrules_8h.html#a5f8a89ec2b58323e9f96ac0bf2c13ab5

[sffc]

2021-06-10:

• @Manishearth - We don't have vectors over FFI yet. You should start with the Rust side though. But on FFI, there are two ways to do this: (1) return an iterator, similar to what we do for strings (Writeable), or (2) return a custom type that wraps Vec and makes it possible to read from Vec on the other side.
• @sffc - Should we be concerned about no_std with this API? Returning an enumeration might be better. Or an enum set?
• @Manishearth - Or an array/struct of bools. The struct should have 6 fields, all bool.
• @sffc - Should we use this as an opportunity to put together our story for enum set?
• @Manishearth - I think it would make sense to have a proc macro that generates the enum set for an enum. Or a regular macro. We could use a bitvec, too (https://docs.rs/bitvec/0.22.3/bitvec/array/struct.BitArray.html). But let's start with an array of bools and make it better later.