Provide epoch nanosecond computation from named time zone and datetime fields

[hsivonen]

Temporal requires an implementation of GetNamedTimeZoneEpochNanoseconds, which takes a named time zone and the components of a datetime as distinct numbers and computes the corresponding moment(s) in epoch nanoseconds.

The computation should be able to return multiple epoch nanosecond moments or none when the datetime fields designate an ambiguous or non-existing moment due to time zone transitions.

[sffc]

CC @leftmostcat @nordzilla