Create `fs` module

[gurry]

Currently we have a separate virtfs_efi crate which offers a way to create a virtual, possibly in-memory file system. It does this by implementing the SIMPLE_FILE_SYSTEM protocol. Perhaps it's a better idea to merge this crate into efi crate under a module called fs. After that we can change its behaviour so that instead of taking io::Read objects it takes interfaces that define a file store. The file store should expose ways to open a file and read a file. The fs module will then wrap this into an implementation of SIMPLE_FILE_SYSTEM protocol.

Alternatively we can the above changes but keep the code in virtfs_efi. Also may be we can change the crate name to fs_efi because there's nothing "virtual" about the above mentioned new design.