Added clarification on buffer alignment.

In v2, the lookahead_buffer was changed from requiring 4-byte alignment
to requiring 8-byte alignment. This was not documented as well as it
could be, and as FabianInostroza noted, this also implies that
lfs_malloc must provide 8-byte alignment.

To protect against this, I've also added an assert on the alignment of
both the lookahead_size and lookahead_buffer.

found by FabianInostroza and amitv87

lfs_util.h

@@ -192,6 +192,7 @@ static inline uint32_t lfs_tobe32(uint32_t a) {
 uint32_t lfs_crc(uint32_t crc, const void *buffer, size_t size);
 
 // Allocate memory, only used if buffers are not provided to littlefs
+// Note, memory must be 64-bit aligned
 static inline void *lfs_malloc(size_t size) {
 #ifndef LFS_NO_MALLOC
     return malloc(size);