Added support for handling corrupted blocks

This provides a limited form of wear leveling. While wear is
not actually balanced across blocks, the filesystem can recover
from corrupted blocks and extend the lifetime of a device nearly
as much as dynamic wear leveling.

For use-cases where wear is important, it would be better to use
a full form of dynamic wear-leveling at the block level. (or
consider a logging filesystem).

Corrupted block handling was simply added on top of the existing
logic in place for the filesystem, so it's a bit more noodly than
it may have to be, but it gets the work done.

tests/test_dirs.sh

@@ -124,6 +124,7 @@ tests/test.py << TEST
 TEST
 
 echo "--- Directory remove ---"
+# TESTING HERE
 tests/test.py << TEST
     lfs_mount(&lfs, &cfg) => 0;
     lfs_remove(&lfs, "potato") => LFS_ERR_INVAL;