Add mechanism for linking resource keys with data structs

[sffc]

There is currently no documentation that links resource keys, like icu_decimal::provider::key::SYMBOLS_V1, to data structs, like icu_decimal::provider::DecimalSymbolsV1. We should figure out a way to document and enforce this mapping.

[sffc]

• @nordzilla: Could we use associated constants? icu_decimal::provider::DecimalSymbolsV1::key::SYMBOLS_V1

[sffc]

Here's the approach I'd like to try. I can add a new field, hidden behind a feature flag, to ResourceKey: type_id: core::any::TypeId. This field would store the type information of the struct that we expect to see when the key is used. I can then check at runtime whether the type signature matches the key.

However, I am hitting rust-lang/rust#77125, because we always create ResourceKey in a const context. I'm therefore going to consider this issue blocked on upstream stabilizing const TypeId::of().