Restructured directory code

After quite a bit of prototyping, settled on the following functions:
- lfs_dir_alloc  - create a new dir
- lfs_dir_fetch  - load and check a dir pair from disk
- lfs_dir_commit - save a dir pair to disk
- lfs_dir_shift  - shrink a dir pair to disk
- lfs_dir_append - add a dir entry, creating dirs if needed
- lfs_dir_remove - remove a dir entry, dropping dirs if needed

Additionally, followed through with a few other tweaks

lfs_util.c

@@ -7,7 +7,7 @@
 #include "lfs_util.h"
 
 
-uint32_t lfs_crc(const void *buffer, lfs_size_t size, uint32_t crc) {
+uint32_t lfs_crc(uint32_t crc, lfs_size_t size, const void *buffer) {
     static const uint32_t rtable[16] = {
         0x00000000, 0x1db71064, 0x3b6e20c8, 0x26d930ac,
         0x76dc4190, 0x6b6b51f4, 0x4db26158, 0x5005713c,