Use N-bit lengths for VZV IndexN format types (#5594)

It is impossible for an IndexN array to need more than a length integer
of size N, anyway, the max index is always `>=` the length.


Part of #5523

Builds on #5593


We could in theory just have a `VZVFormatCombo<Index, Len>` type that
allows free selection, however I'm trying to keep this minimal. Overall
the main use case for that is picking things like "a small array of
;argely-sized elements" and we could just expose Index16Len8 for that. I
can see that being useful for things like
#5580, though it also feels
like a data microoptimization.


The "total" lines in fingerprints.csv are interspersed in giant diffs,
and this basically only gets a max of 2-6 byte wins per data, but the
overall data size went down by 200KB. Not amazing, not terrible.

```rust
[18:26:22] मanishearth@manishearth-glaptop2 ~/dev/icu4x/provider/data ^_^ 
$ rg total | awk '{ gsub(/B,/, "", $3); s +=$3} END{print s}' 
23501369
[18:26:08] मanishearth@manishearth-glaptop2 ~/dev/icu4x/provider/data ^_^ 
$ rg total | awk '{ gsub(/B,/, "", $3); s +=$3} END{print s}' 
23391499
```

CHANGELOG.md

@@ -38,6 +38,7 @@
     - `zerotrie`
     - `zerovec`
       - This release has multiple changes that affect the bit representation of various types. Do not update to this release if you wish to retain stable data formats.
+        - Change the `VarZeroVecFormat` values shipped by default to use the same index and length width. This breaks data layout for all `VarZeroVec`s. (https://github.com/unicode-org/icu4x/pull/5594)
         - Optimize `MultiFieldsULE` to not store a length anymore. This breaks data layout for any `#[make_varule]`-using struct with multiple variable-sized fields. (https://github.com/unicode-org/icu4x/pull/5593)
     - `writeable`
 

components/plurals/src/provider.rs

@@ -1073,10 +1073,10 @@ fn test_serde_nonsingleton_roundtrip() {
     assert_eq!(
         postcard_bytes,
         &[
-            16,   // Postcard header
+            14,   // Postcard header
             0x80, // Discriminant
             3, b'a', b'b', b'c', // String of length 3
-            1, 0, 0, 0, 0, 0, // VarZeroVec of length 1
+            1, 0, 0, 0, // VarZeroVec of length 1
             0x10, b'd', b'e', b'f', b'g' // Plural category 1 and string "defg"
         ]
     );

components/plurals/src/rules/runtime/ast.rs

@@ -573,7 +573,7 @@ mod test {
         let vzv = VarZeroVec::<_>::from(relations.as_slice());
         assert_eq!(
             vzv.as_bytes(),
-            &[1, 0, 0, 0, 0, 0, 192, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0]
+            &[1, 0, 0, 0, 192, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0]
         );
     }
 }

provider/blob/benches/auxkey_bench.rs

@@ -293,7 +293,7 @@ fn make_blob_v2() -> Vec<u8> {
     put_payloads::<MarkerD>(&mut exporter);
     exporter.close().unwrap();
     drop(exporter);
-    assert_eq!(blob.len(), 32982);
+    assert_eq!(blob.len(), 32980);
     assert!(blob.len() > 100);
     blob
 }

provider/data/casemap/fingerprints.csv

@@ -1,2 +1,2 @@
-props/casemap@1, <singleton>, 22515B, 22428B, ff76a17649a34363
-props/casemap_unfold@1, <singleton>, 976B, 932B, 85d63de2fdea5a3d
+props/casemap@1, <singleton>, 22513B, 22426B, a4a125633510e06d
+props/casemap_unfold@1, <singleton>, 972B, 928B, b861f4456b3907f