Added internal check of data written to disk

Before, the littlefs relied on the underlying block device
to report corruption that occurs when writing data to disk.
This requirement is easy to miss or implement incorrectly, since
the error detection is only required when a block becomes corrupted,
which is very unlikely to happen until late in the block device's
lifetime.

The littlefs can detect corruption itself by reading back written data.
This requires a bit of care to reuse the available buffers, and may rely
on checksums to avoid additional RAM requirements.

This does have a runtime penalty with the extra read operations, but
should make the littlefs much more robust to different implementations.

lfs.h

@@ -32,7 +32,7 @@ typedef uint32_t lfs_block_t;
 enum lfs_error {
     LFS_ERR_OK      = 0,    // No error
     LFS_ERR_IO      = -5,   // Error during device operation
-    LFS_ERR_CORRUPT = -77,  // Corrupted
+    LFS_ERR_CORRUPT = -52,  // Corrupted
     LFS_ERR_NOENT   = -2,   // No directory entry
     LFS_ERR_EXISTS  = -17,  // Entry already exists
     LFS_ERR_NOTDIR  = -20,  // Entry is not a dir