Use N-bit lengths for VZV IndexN format types

[Manishearth]

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.

[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

[sffc]

It's not super common that Index8 is big enough, but it is quite common that Len8 is big enough. I feel like Index8Len16 would be one of the most commonly used settings.

[Manishearth]

@sffc we can expose it easily in a followup, yeah. I think Index16Len8 is definitely more useful than Index8 and we should consider looking at where we use Index8 now.

[sffc]

Suggestion: Why not just use size_of::<Self>() which is const?

[Manishearth]

I found it makes the code easier to read but I'm open to that being a followup

[Manishearth]

basically mem::size_of::<F::Index> feels clunkier than F::Index::SIZE in the situations it gets used in.

however in my next PR I'm removing METADATA_WIDTH and that may make the expressions simple enough that a sizeof won't be a big deal