rust: Add a Timestamp struct instead of abusing Duration
`SystemTime` would be the standard library type semantically appropriate
instead of `Duration`.
But since the value is coming from Python as a plain integer and used in
dirstate packing code as an integer, let’s make a type that contains a single
integer instead of using one with sub-second precision.
Differential Revision: https://phab.mercurial-scm.org/D10485
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