Replace / expand `prefix` to allow using a generic function for rewriting paths about rust-embed

[WesleyAC]

One thing I haven't considered too much here is the implementation of the debug mode — doing something like this would require watching the directory for changes to recalculate the names, since there's no way to reverse the rename function (unlike prefix). That's potentially a significant amount of complexity, but I do think it would be worthwhile.

[pyrossh]

Interesting Idea. Thats how it is generally handled by web bundlers like webpack/rollup in meta-frameworks when serving immutable assets. They would compile the assets to include the hash in the name. So it should work out of the box for those tools. For other cases where it needs to be manually done this feature would be useful.

Does it make sense to make this a setting or a feature so that it can be done automatically since many use cases involves serving web assets. I haven't seen passing a function reference as an attribute to custom derive before. Might need to test it out.
For the 2nd point I don't know why you would want to do it in debug mode unless you want to cache there as well.

The easiest solution I can think of is adding another key in the BTreeMap with the format $path/$filename.$hash.$ext pointing to the file.