SCP-2596: New posixTimeRange <-> SlotRange logic

[sjoerdvisscher]

Now posixTimeRangeToContainedSlotRange returns the largest slot range that is entirely contained by the time range, and slotRangeToPOSIXTimeRange takes exclusive bounds into account.

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[michaelpj]

Can we give these things explicit names that make it clear what's happening? e.g. slotRangeWithinPosixTimeRange/posixTimeRangeContainingSlotRange or something? I do think it's misleading having a bare posixTimeRangeToSlotRange that sounds like it's "perfect", when in fact there are a few options.

[michaelpj]

(Feel free to bikeshed on names, I think it's hard to give a good one here...)