Export an Encodeable instead of a VarULE and use Index32

[sffc]

Breaking change in icu_provider extracted from #7309.

The trait unfortunately wasn't implemented with EncodeAsVarULE in mind, so this is a breaking change in icu_provider in a trait that isn't used by anything except hello_world (which lives in that crate) and is only implemented by macros that are versioned along with the crate. Do we care? Please speak up.

[Manishearth]

Yeah, this makes sense. Unfortunate oversight. I'm wary of the breaking change, but it does not seem to be a real problem yet.

[sffc]

cargo-semver-check suggested that I add a default for the associated type, but when I do that, I get an error saying that associated type defaults are not stable. rust-lang/rust#29661

[sffc]

Here's the semver check report

    Checking icu_provider v2.1.1 -> v2.1.1 (no change; assume patch)
     Checked [   0.027s] 184 checks: 180 pass, 4 fail, 0 warn, 16 skip

--- failure trait_associated_type_added: non-sealed public trait added associated type without default value ---

Description:
A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/trait_associated_type_added.ron

Failed in:
  trait associated type icu_provider::ule::MaybeEncodeAsVarULE::EncodeableStruct in file /home/runner/work/icu4x/icu4x/provider/core/src/varule_traits.rs:36

--- failure trait_method_added: pub trait method added ---

Description:
A non-sealed public trait added a new method without a default implementation, which breaks downstream implementations of the trait
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/trait_method_added.ron

Failed in:
  trait method icu_provider::ule::MaybeEncodeAsVarULE::maybe_as_encodeable in file /home/runner/work/icu4x/icu4x/provider/core/src/varule_traits.rs:41

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#major-any-change-to-trait-item-signatures
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/trait_method_missing.ron

Failed in:
  method maybe_encode_as_varule of trait MaybeEncodeAsVarULE, previously in file /home/runner/work/icu4x/icu4x/semver-checks/target/semver-checks/git-origin_main/d6f790cc393030a5d1382de3f18bbc857d401ae6/provider/core/src/varule_traits.rs:35

--- failure trait_no_longer_dyn_compatible: trait no longer dyn compatible ---

Description:
Trait is no longer dyn compatible, which breaks `dyn Trait` usage.
        ref: https://doc.rust-lang.org/stable/reference/items/traits.html#object-safety
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/trait_no_longer_dyn_compatible.ron

Failed in:
  trait MaybeEncodeAsVarULE in file /home/runner/work/icu4x/icu4x/provider/core/src/varule_traits.rs:34

     Summary semver requires new major version: 4 major and 0 minor checks failed

If I switch the function name back to what it was before and add a default associated type (not possible in stable Rust), I think 3/4 of those errors go away.

But I'm going to merge this for now since @Manishearth approved it.