Split DateTime symbols data into length-based chunks

[Manishearth]

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Part of https://github.com/unicode-org/icu4x/issues/3413

Currently DateTimeSymbols data contains data for every length of symbol data, regardless of which is requested. We shoudl slice it down into MonthSymbols (which can be long/short/medium/full), WeekSymbols, etc and load just what is needed during data loading.

This is likely a 2.0 candidate (https://github.com/unicode-org/icu4x/issues/2856). This will need some investigation.

[sffc]

Partly duplicate of #3865

[Manishearth]

This is done with neo dateformatter.