crtime: add (Mono).Add

[stevendanna]

This may make it a bit easier to adopt Mono in libraries without forcing us to immediately update all callers.

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This change is 

[stevendanna]

Happy to drop the interface definition from the PR if that is preferred.

[RaduBerinde]

Yeah, honestly it's a bit strange to have an interface defined that is not used anywhere.

otherwise

@RaduBerinde reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @stevendanna)

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crtime/monotonic.go line 50 at r1 (raw file):

}

// Add returns the time.Mono m+d.

time. is not right, mayne just say "the moment m+d"

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crtime/monotonic.go line 52 at r1 (raw file):

// Add returns the time.Mono m+d.
func (m Mono) Add(d time.Duration) Mono {
	return Mono(time.Duration(m) + d)

OCD nit but m + Mono(d) is more intuitive to me