add derived Copy impls for Dir, DirEntry, and File

[jbr]

As this only adds behavior, this is semver-patch

[Michael-F-Bryan]

I'm reluctant to derive Copy for these types because it means we can't add compression (#14) later on without causing breakage.

Is there a particular use case you had in mind that requires Dir: Copy?

[jbr]

In this case, I was mostly just surprised they weren't already Copy. I was doing a bunch of cloning to work around #73 when I ran into this.

[Michael-F-Bryan]

I'm curious why you needed to clone the File handles or were running into lifetime issues. Do you have any simple examples for where the lack of 'static lifetimes causes problems?

[Michael-F-Bryan]

I'm going to close this for now seeing as it would prevent future optimisations like compression.