RFC: introduce notations for `Duration` and `Timestamp` and remove dangerous `OfNat` instances

Proposal

We currently provide OfNat Duration and OfNat Timestamp instances which turn natural numbers into seconds and seconds since Unix epoch, respectively. Keeping with the design of "make incorrect conversions impossible and make dubious conversions explicit", we should change this as follows:

• Introduce notation for Duration types (this was considered during initial implementation, but was postponed to reduce the size of the PR).
• Add Timestamp.ofDurationSinceUnixEpoch (this is just the constructor).
• Remove the OfNat Duration and OfNat Timestamp instances.
• Possibly introduce notation for Timestamp that allows entering a timestamp with time zone in ISO format without having to go through ZonedDateTime.

Impact

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Start by locating the Duration and Timestamp definitions and their current OfNat instances. Review how existing notation and ZonedDateTime APIs are implemented before assessing the proposed Duration notation and Timestamp.ofDurationSinceUnixEpoch constructor. Done would require an agreed design covering the notation, constructor, removal of both instances, and whether ISO timestamp notation is included.