Added optional block-level caching

This adds caching of the most recent read/program blocks, allowing
support of devices that don't have byte-level read+writes, along
with reduced device access on devices that do support byte-level
read+writes.

Note: The current implementation is a bit eager to drop caches where
it simplifies the cache layer. This layer is already complex enough.

Note: It may be worthwhile to add a compile switch for caching to
reduce code size, note sure.

Note: This does add a dependency on malloc, which could have a porting
layer, but I'm just using the functions from stdlib for now. These can be
overwritten with noops if the user controls the system, and keeps things
simple for now.

lfs_util.h

@@ -8,6 +8,7 @@
 #define LFS_UTIL_H
 
 #include "lfs_config.h"
+#include <stdlib.h>
 
 
 // Builtin functions
@@ -34,5 +35,4 @@ static inline int lfs_scmp(uint32_t a, uint32_t b) {
 uint32_t lfs_crc(uint32_t crc, lfs_size_t size, const void *buffer);
 
 
-
 #endif