uefi: fs: Implement exists

Also adds the initial file abstractions.

The file opening algorithm is inspired from UEFI shell. It starts by
classifying if the Path is Shell mapping, text representation of device
path protocol, or a relative path and converts into an absolute text
representation of device path protocol.

After that, it queries all handles supporting
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL and opens the volume that matches the
device path protocol prefix (similar to Windows drive). After that, it
opens the file in the volume using the remaining pat.

It also introduces OwnedDevicePath and BorrowedDevicePath abstractions
to allow working with the base UEFI and Shell device paths efficiently.

DevicePath in UEFI behaves like an a group of nodes laied out in the
memory contiguously and thus can be modeled using iterators.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

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