implement symlinks for virtual-fs

[ijsnow]

Description

This is PR implements symlinks for the virtual-fs crate. It is a WIP and I'd appreciate any feedback/confirmation I'm heading in a direction that would get the implementation accepted! This would close https://github.com/wasmerio/wasmer/issues/3232.

The main chunk of work I'd like reviewed is for mem_fs::FileSystem. I based the logic for the symlink functionality in the memory file system based on functionality I found playing around with std::os::unix::fs::symlink in this gist.

Remaining work outside of any feedback is to implement symlink for any of the file systems using webc::v1::Volume as I can't seem to find the repository containing the crate (is pirita private?).

[ptitSeb]

pirita is pivate for now, but that will change (very) soon. Why using a feature for the symlink?

The PR looks ok for now, appart for the symlink behind a feature. And I see some tests already, which is good.

@Michael-F-Bryan if you can also have a look.