Added atomic move using dirty tag in entry type

The "move problem" has been present in littlefs for a while, but I haven't
come across a solution worth implementing for various reasons.

The problem is simple: how do we move directory entries across
directories atomically? Since multiple directory entries are involved,
we can't rely entirely on the atomic block updates. It ends up being
a bit of a puzzle.

To make the problem more complicated, any directory block update can
fail due to wear, and cause the directory block to need to be relocated.
This happens rarely, but brings a large number of corner cases.

---

The solution in this patch is simple:
1. Mark source as "moving"
2. Copy source to destination
3. Remove source

If littlefs ever runs into a "moving" entry, that means a power loss
occured during a move. Either the destination entry exists or it
doesn't. In this case we just search the entire filesystem for the
destination entry.

This is expensive, however the chance of a power loss during a move
is relatively low.

Makefile

@@ -32,7 +32,7 @@ size: $(OBJ)
 
 .SUFFIXES:
 test: test_format test_dirs test_files test_seek test_parallel \
-	test_alloc test_paths test_orphan test_corrupt
+	test_alloc test_paths test_orphan test_move test_corrupt
 test_%: tests/test_%.sh
 	./$<