unicode-org / icu4x / 11041072971

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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
https://github.com/unicode-org/icu4x/pull/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
```

83 of 89 new or added lines in 2 files covered. (93.26%)

55024 of 72682 relevant lines covered (75.71%)

589323.0 hits per line