Ability to construct virtual `Dir`s to mount files and subdirectories into

Specifically with the option for those files/dirs to just be paths that don’t need to be opened when mounted. Note: When I say “mount” here this would only be internal to the `Dir` struct and not actual mounts on the underling filesystem. Similar to `Pool`’s `insert` methods.

Would particularly be interesting to create a root `Dir` which would just be `/` on Unix but on Windows a folder exposing all drive letters as subfolders. I understand that this specific use case may be controversial, as it seems to go against the spirit of this crate. But I believe the opposite to be true: The great thing about this API is that it doesn’t just have one or two layers (as more basic security systems), but arbitrarily many as each `Dir` recursively can be used to create narrower views. So I don’t think one global directory as the base to craft more specialised capabilities would inherently be a bad idea.

Based originally on thinking in https://github.com/bytecodealliance/wasmtime/issues/8552 to solve the problem of Windows (unlike Unix) not having a single root folder but essentially one for each volume, which makes `cap-std` difficult to use in some contexts.

I apologise should what I propose already be possible, in a quick look into the docs I haven’t figured out a way to do this.