POSIXTime.
This eliminates old-time, which has been deprecated for a while, from
our dependencies.
This introduces a slight incompatibility, as a small number of functions
now take/return POSIXTime instead of ClockTime. Generally you will be
using the current time, in which case you should use getPOSIXTime from
Data.Time.Clock.POSIX instead of getClockTime.
utcTimeToClockTime has been removed, as it is now equivalent to
utcTimeToPOSIXSeconds from Data.Time.Clock.POSIX.
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