time module

**Describe the problem/challenge you have**

even though ytt does not have access to current time, it's handy to be able to work with date/time formats. for example i'd like to parse two iso8601 timestamps, compare them or increment/decrement them.

Example of a timestamp i'd like to work with (from k8s): `creationTimestamp: "2021-06-11T05:00:49Z"` and want to be able to add/substract durations (as desribed by go's time.Duration)

**Describe the solution you'd like**

i think exposing time functionality as provided by go will be the easiest way to go about this. lets discuss the api for time module in this issue.

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The issue names no files, tests, or entry points. Start by reviewing ytt's existing module conventions and the Go time package, especially ISO 8601 parsing and time.Duration behavior. Done would require an agreed API for parsing, comparing, and adjusting timestamps, followed by implementation and tests.

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