Add BasicWithKeyFileAttributeView interface

This new interface is similar to `BasicFileAttributeView` except it
gives implementations a hint that the fileKey() should be acquired
even at some performance cost.

`FileTreeWalker` uses this new interface to request a file key
in addition to regular file attributes so that file equality can
be efficiently performed when checking for loops during file
tree traversal.

This makes `FileTreeWalker` about 2x faster when traversing non
trivial file system trees with the FOLLOW_LINKS option.

Change-Id: Ifa01dc615dec3343bd0993b742c6401d69f2b5eb
5 files changed