[RFC] add Duration::as_millis

[durka]

This PR adds a function to parallel Duration::from_millis, to get the milliseconds back out. It reduces friction when using other APIs that deal in milliseconds, while allowing Rust code to use Duration internally.

Example: I wanted to take in a Duration and pass it as milliseconds to an FFI function that expects an int, and it's messy to implement this function outside of std.

[rust-highfive]

r? @brson

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[nagisa]

I believe this was delayed so u128/i128 as a return value could be used (EDIT: no it was to_nanos). Current implementation will panic for large durations.

See also #28404 and #35868

[durka]

OK, from rust-lang/rfcs#1547 it basically sounds like the decision was stalemate. I'll just use chrono.