Look into replacing flat DateTimeSymbols arrays with VarZeroVecs

[Manishearth]

Part of #3413

Currently DateTimeSymbols data has a couple of fixed-length arrays. These are big on the stack; it's worth replacing them with VZVs. We'd need to measure if this is actually a major win.

This can be done backwards compatibly but we may wish to consider doing it in 2.0 (#2856).

[sffc]

Likely to be handled as part of #3865

[sffc]

This is done. The neo datetime names now use one of the following three structs.

https://unicode-org.github.io/icu4x/rustdoc/icu_datetime/provider/neo/struct.LinearNamesV1.html

https://unicode-org.github.io/icu4x/rustdoc/icu_datetime/provider/neo/enum.MonthNamesV1.html

https://unicode-org.github.io/icu4x/rustdoc/icu_datetime/provider/neo/enum.YearNamesV1.html