Still missing quite a bit, but rudimentary inlined-trees are now working

And by working, I mean you can create inlined trees, just don't
compact/split/move/etc anything. But this does outline the path files
take when writing buffers into inlined trees.

"Inlined trees" in littlefs are entire small rbyd trees embedded as
secondary trees in an mdir's main rbyd tree. When fetching, we can
indicate if a given trunk belongs to the main tree or secondary tree by
setting one of the unused mode bits in the trunk's tag, now called the
"deferred" bit. This bit doesn't need to be included in the alt's "key"
field, so there's no issue with it conflicting with the alt's mode bits.

This requires a bit of tweaking lfsr_rbyd_fetch, since it needs to fall
back to the previous trunk if it discovers the most recent trunk belongs
to an inlined tree. But as a benefit we can leverage the full power of
rbyds in inlined files, including holes, partial updates, etc.

One downside is it looks like these inlined trees may involve more work
in maintining their state correctly, since they need to be sort of
"brought along" when mdirs are compacted, even if they don't actually
have a reference in the mdir yet. But the sheer amount of flexibility
this gives inlined files may make this overhead worth it.

scripts/dbglfs.py

@@ -32,6 +32,7 @@ TAG_REG         = 0x0202
 TAG_DIR         = 0x0203
 TAG_STRUCT      = 0x0300
 TAG_INLINED     = 0x0300
+TAG_TRUNK       = 0x0304
 TAG_BLOCK       = 0x0308
 TAG_BTREE       = 0x030c
 TAG_MDIR        = 0x0311
@@ -178,6 +179,7 @@ def tagrepr(tag, w, size, off=None):
     elif (tag & 0xff00) == TAG_STRUCT:
         return '%s%s %d' % (
             'inlined' if tag == TAG_INLINED
+                else 'trunk' if tag == TAG_TRUNK
                 else 'block' if tag == TAG_BLOCK
                 else 'btree' if tag == TAG_BTREE
                 else 'mdir' if tag == TAG_MDIR