Make duration constructor functions `const`.

[quadrupleslap]

Is there anything stopping the functions from_millis, from_secs and new in std::time::Duration from being constant?

[nagisa]

You cannot use a const fn with stable compiler as a constant function, so for most purposes making these functions const now is not really useful.

[sfackler]

I'm not totally sure I understand that, @nagisa - stable users of the compiler will see no change and people on nightlies will be able to take advantage of the const-ness.

[tbu-]

Now impossible until we allow panics in const functions: #33072.

[ticki]

#34918

[dtolnay]

This is going to depend on what happens with rust-lang/rfcs#1383. Let's continue to track this as part of #24111 as Alex commented in #33033 (comment):

Adding a const fn is a serious API commitment that we may not always be able to uphold (e.g. #33072). In the meantime const fn is still unstable, and when stabilized we will likely want to perform a pass over the standard library to audit for functions which are const-compatible at that time.