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.

tests/test_rbyd.toml

@@ -0,0 +1,28 @@
+
+# Test this inner rbyd data-structure
+
+[cases.rbyd_create]
+in = 'lfs.c'
+code = '''
+    lfs_t lfs;
+    lfs_init(&lfs, cfg) => 0;
+
+    lfs_rbyd_t rbyd = {
+        .block = 0,
+        .trunk = 0,
+        .noff = 0,
+        .rev = 1,
+        .crc = 0,
+        .count = 0,
+        .erased = true,
+    };
+
+    //lfs_rbyd_commit(&lfs, &rbyd, NULL) => 0;
+
+    uint32_t data = 0xa5dfa5df;
+    lfs_rbyd_commit(&lfs, &rbyd,
+            LFS_MKRATTR(UATTR, 1, 0, &data, 4,
+            LFS_MKRATTR(UATTR, 2, 0, &data, 4,
+            NULL))) => 0;
+
+'''