Initial groundwork for rbyd trees

- primitive lfs_rbyd_fetch
- primitive lfs_rbyd_commit
- tag reading/progging and encoding machinery

The tag encoding scheme here uses pairs of leb128s, encoding either
a normal tag:

  iiii iiiiiii iiiiiTT TTTTTTt ttttt0v
                   ^--------^------^-^- 16-bit id
                            '------|-|- 8-bit type2
                                   '-|- 6-bit type1
                                     '- valid bit
  llll lllllll lllllll lllllll lllllll
                                     ^- n-bit length

Or an alt pointer:

  wwww wwwwwww wwwwwww wwwwwww wwwcd1v
                                 ^^^-^- 28-bit weight
                                  '|-|- color bit
                                   '-|- direction bit
                                     '- valid bit
  jjjj jjjjjjj jjjjjjj jjjjjjj jjjjjjj
                                     ^- n-bit jump

Note that two bits overlap the alt pointer dir/color encoding, this
is actually not a problem at all since some tags (crcs/fcrcs) don't
participate in the rbyd tree and can use these bits.

There's a number of benefits to using leb128s, which should probably
be written about, most notably is the abstraction of the device's
word-size. The "n-bits" above can be whatever word size works on the
device, trading off code-size for storage capabilities without breaking
compatibility with other devices. This will eventually be negotiated via
the superblock.

lfs.h

@@ -84,6 +84,7 @@ enum lfs_error {
     LFS_ERR_NOMEM       = -12,  // No more memory available
     LFS_ERR_NOATTR      = -61,  // No data/attr available
     LFS_ERR_NAMETOOLONG = -36,  // File name too long
+    LFS_ERR_OVERFLOW    = -75,  // Value too large for defined data type
 };
 
 // File types
@@ -326,6 +327,16 @@ typedef struct lfs_cache {
     uint8_t *buffer;
 } lfs_cache_t;
 
+typedef struct lfs_rbyd {
+    lfs_block_t block;
+    lfs_off_t trunk;
+    lfs_off_t noff;
+    uint32_t rev;
+    uint32_t crc;
+    uint16_t count;
+    bool erased;
+} lfs_rbyd_t;
+
 typedef struct lfs_mdir {
     lfs_block_t pair[2];
     uint32_t rev;