Added some simple mtree benchmarks

It's interesting to note the different performance characteristics of
purely CoW btrees vs our mutable mtree.

The main downside of our mtree is the need to fetch leaf mdirs. This
fetch is expensive, and can be avoided in CoW btrees by storing the
trunk in each branch's parent.

On the other hand, btrees need to propagate all changes upwards to the
root.

An interesting takeaway is that a sort of mdir-trunk cache may be a very
interesting optimization for relatively little RAM cost. This may be
something to explore in the future.

tests/test_mtree.toml

@@ -1,3 +1,7 @@
+# maximize lookahead buffer, we don't actually gc so we only get one pass
+# of the disk for these tests
+defines.LOOKAHEAD_SIZE = 'BLOCK_COUNT / 8'
+
 # test a single mroot
 [cases.test_mtree_one_mroot]
 code = '''