Changed mtree to be weighted by mdir upper bound

More on this when explaining compressed mids, but basically the idea is
instead of just storing all mdirs in our mtree as single element
entries, store each mdir in as a weighted entry, where the weight is a
known upper bound on the possible number of mid entries in a single
mdir.

With the current mid representation, this just complicates things
without much benefits. But with compressed mids it allows us to lookup
mdirs with the mid directly, and avoid decoding the bid from the mid in
some cases.

The mid-per-mdir upper bound is derived from the block size. We know:

1. Each tag needs <=2 alts+null with our current compaction strategy
2. Each tag/alt encodes to a minimum of 4 bytes

This gives us ~4*4 or ~16 bytes per mid at minimum. If we cram an mdir
with the smallest possible mids, this gives us at most ~block_size/16
mids in a single mdir before the mdir runs out of space.

Note we can't assume ~1/2 block utilization here, as an mdir may
temporarily fill with more mids before compaction occurs.

lfs.h

@@ -517,13 +517,14 @@ typedef struct lfs {
     lfs_size_t attr_max;
 
     // begin lfsr things
-    lfsr_mdir_t mroot;
-    lfsr_btree_t mtree;
-
     lfsr_grm_t grm;
     uint8_t pgrm[LFSR_GRM_DSIZE];
     uint8_t dgrm[LFSR_GRM_DSIZE];
 
+    uint8_t mrid_bits;
+    lfsr_mdir_t mroot;
+    lfsr_btree_t mtree;
+
     // linked-lists of opened mdirs, we keep a separate linked-list
     // for each type since these need to be handled a bit differently
     lfsr_openedmdir_t *opened[2];