Initial fs implementation for UEFI

- Only implementing basic file stuff for now.
- Not implementing path::absolute right now since absolute UEFI paths
  will not be recognized as absolute path anyway.
- Supports both UEFI shell mapping and device path text representation.
- Follows closely UEFI shell file implementations.

Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>

Author: Ayush1325

Diff for: library/std/src/lib.rs

@@ -349,6 +349,7 @@
 #![feature(portable_simd)]
 #![feature(ptr_as_uninit)]
 #![feature(ptr_mask)]
+#![feature(ptr_metadata)]
 #![feature(random)]
 #![feature(slice_internals)]
 #![feature(slice_ptr_get)]