Partial implementation of a rudimentary mtree

This became surprisingly tricky.

The main issue is knowing when to split mdirs, and how to determine
this without wasting erase cycles.

Unlike splitting btree nodes, we can't salvage failed compacts here. As
soon as the salvage commit is written to disk, the commit becomes immediately
visibile to the filesystem because it still exists in the mtree. This is
a problem if we lose power.

We're likely going to need to implement rbyd estimates. This is
something I hoped to avoid because it brings in quite a bit of
complexity and might lead to an annoying amount of storage waste since
our estimates will need to be conservative to avoid unrecoverable
situations.

---

Also changed the on-disk btree/branch struct to store a copy of the weight.

This was already required for the root of the btree, requiring the
weight to be stored in every btree pointer allows better code
deduplication at the cost of some redundancy on btree branches, where
the weight is already implied by the rbyd structure.

This weight is usually a single byte for most branches anyways.

This may be worth revisiting at some point to see if there's any other
unexpected tradeoffs.

scripts/dbgbtree.py

@@ -367,9 +367,10 @@ def main(disk, root=0, *,
                 # is it another branch? continue down tree
                 if struct_tag == TAG_BRANCH and (
                         depth is None or depth_ < depth):
-                    trunk, d1 = fromleb128(struct_)
-                    block, d2 = fromleb128(struct_[d1:])
-                    crc = fromle32(struct_[d1+d2:])
+                    w, d1 = fromleb128(struct_)
+                    trunk, d2 = fromleb128(struct_[d1:])
+                    block, d3 = fromleb128(struct_[d1+d2:])
+                    crc = fromle32(struct_[d1+d2+d3:])
                     rbyd = Rbyd.fetch(f, block_size, block, trunk)
 
                     # corrupted? bail here so we can keep traversing the tree