Added allocation randomization for dynamic wear-leveling

This implements the second step of full dynamic wear-leveling, block
allocation randomization. This is the key part the uniformly distributes
wear across the filesystem, even through reboots.

The entropy actually comes from the filesystem itself, by xoring
together all of the CRCs in the metadata-pairs on the filesystem. While
this sounds like a ridiculous operation, it's easy to do when we already
scan the metadata-pairs at mount time.

This gives us a random number we can use for block allocation.
Unfortunately it's not a great general purpose random generator as the
output only changes every filesystem write. Fortunately that's exactly
when we need our allocator.

---

Additionally, the randomization created a mess for the testing
framework. Fortunately, this method of randomization is deterministic.
A very useful property for reproducing bugs.

lfs.h

@@ -382,6 +382,7 @@ typedef struct lfs {
 
     lfs_block_t root[2];
     lfs_mlist_t *mlist;
+    uint32_t seed;
 
     lfs_global_t globals;
     lfs_global_t locals;