Extend const_convert for Cow on From<&str> and From<String>

[nipunn1313]

These implementations are trivially const - since they only create the enum structure
without running any non-const code.

const_convert is tracked by #88674

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[scottmcm]

@rust-lang/libs What's the status of impl const these days?

I remember seeing a decision to stop doing some of the trait-level constification because that's not on stable yet, but I forget if it was just ~const/?const or if it applied to these too.

[thomcc]

I'm not sure a decision has been made.

CC @rust-lang/wg-const-eval (who might have an opinion too).

[fee1-dead]

This should be okay. We used to stop adding these to wait for some bugs to be fixed, but currently there aren't any bugs that can affect stable so this is fine.

[nipunn1313]

Hello.
I wanted to check in on this one again!

[oli-obk]

From the current reimplementation status, this is blocked for another few months.

[dtolnay]

Since #110393, impl const Trait for Type is no longer used in the standard library. See also #110395.

I'll close the PR since I don't think this is on track to be landable in this form anytime soon, but after the const trait impls story is better figured out I'd be happy to take a new PR or reopen this one.

Thanks anyway for the PR!