Renamed crystallize_size -> crystal_size

The original name was a bit of a mouthful.

Also dropped the default crystal_size in the test/bench runners
block_size/4 -> block_size/8. I'm already noticing large amounts of
inflation when blocks are fragmented, though I am experimenting with a
rather small fragment_size right now.

Future benchmarks/experimentation is required to figure out good values
for these.

lfs.h

@@ -280,7 +280,7 @@ struct lfs_config {
     // TODO document
     lfs_size_t inline_size;
     lfs_size_t fragment_size;
-    lfs_size_t crystallize_size;
+    lfs_size_t crystal_size;
 };
 
 // File info structure