Associated constants for new duration constructors

[edwardwc]

Add associated constants to the unstable Duration constructors. The tracking issue for these constructors is #120301.

This change aims to make interacting with time more clean and intuitive by providing predefined duration constants.

For example, to use a static for your sleep intervals:

static SLEEP_INTERVAL: Duration = Duration::HOUR;

...

sleep(SLEEP_INTERVAL)

Or with use of a lazy_static

lazy_static! {
    static ref SLEEP_INTERVAL: Duration = 5 * Duration::MINUTE;
}

...

sleep(*SLEEP_INTERVAL)

Or, to use it plainly

sleep(Duration::WEEK)

[rustbot]

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[scottmcm]

Do you have an ACP for these? They're not obvious enough to me that I'd be willing to approve them, even for nightly, without libs-api weighing in.

@rustbot author

[djc]

Adding these makes sense to me -- not sure if they should be part of the same feature?

[edwardwc]

@djc up to you. My thinking is it could be reasonable to bundle these associated constants with their required constructor counterparts as one feature.

[djc]

I don't think it's up to me -- I'm not a libs reviewer.

[Dylan-DPC]

@edwardwc this needs an ACP to be written to proceed. Once you have submitted the ACP, you can mention it here (preferably in the pr description)