Add Promise.sleep to std

[nojaf]

I add let sleep = ms => Promise.make((resolve, _) => setTimeout(resolve, ms)->ignore) to a lot of my projects.

Would like to have this in the standard library.

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commit: 4f5e154

[nojaf]

@cknitt, there's no need for me to hold up the release for this. We can revisit and possibly merge it later.

[cknitt]

Yes, thanks, already proceeding with the release!