Make a funky BoxyDataPayload type for DateTimeFormat

[Manishearth]

Part of #3413

We discussed that DateTimeFormat stack size could be reduced by using a type that behaves like DataPayload, but its yoke variant is boxed. DateTimeFormat would then use it instead for symbols data.

In a future where #3128 happens we can still benefit from this by using a struct-of-borrows type. (effectively a Yoke<SymbolsDataBorrowed, Option<Box<SymbolsData>>> where SymbolsDataBorrowed is a struct full of borrows of the individual types)

In a future where we are no_alloc, the boxing optimization can be disabled in no_alloc mode.

This should be an internal change, though it may result in a new public DataPayload type.

[sffc]

Might be partially fixed by #3865