adds support for swift-concurrency Duration and async closures for Timer measurements

[AndriusA]

extends Timer with support for standard lib Duration type and measurements to relative to Instant on Clocks using Duration

Motivation:

Use of Timer is currently clunky with async code as any durations need to be manually converted to microseconds (et al)

Modifications:

Extends Timer with:

• method to record duration since Clock.Instant on any Clock using Duration
• methods that use static continuous/suspending clock by default with corresponding instant types
• methods to measure duration of an async closure with provided Clock or default ContinuousClock

Result:

Additional Timer methods available for measuring async work

[AndriusA]

note: need to remove Timer.record(duration: Duration) when https://github.com/apple/swift-metrics/pull/133 lands

[FranzBusch]

@AndriusA Can you update this PR so we can start to review it?