FileIdCache: Allow flexible handle instead of direct borrow for file ids

[florian-g2]

Hi,
I suggest changing the return type of the cached_file_id method in the FileIdCache from Option<&FileId> to Option<impl Deref<Target=FileId>>; (breaking change)

This allows trait consumers to choose more freely how to return the FileId.
In a FileIdCache implementation I had to do I was required to return a owned copy of the file id.
The changed return type allows eg. a Cow::Owned or any other new-type implementation that dereferences to a FileId.

This is now doable with the recent MSRV raise to 1.77.

[dfaust]

I'm split on this. FileId is just 256 bytes and implements Copy. So maybe returning Option<FileId> would be easiest. But I'm willing to merge it, if you think it's an improvement.
Please update the change-log as well.