Reverted to mweight -> mleaf_weight and made lfs_t const

We have bleafs (bleaves?) now, so the mleaf name just makes too much
sense. Even though it's used nowhere else outside of mid decoding, and
may be a bit confusing.

After all this time it feels weird to use a const lfs_t parameter, but
that's really what the mid/mleaf functions should take. These functions
are a bit of a special case as lfsr_mleafweight really wants to just be
a constant.

Code size did not change.

lfs.h

@@ -601,7 +601,7 @@ typedef struct lfs {
     // purpose flags field? this has been useful for lfsr_file_t
     bool hasorphans;
 
-    uint8_t mbits;
+    uint8_t mleaf_bits;
     lfsr_mdir_t mroot;
     lfsr_mtree_t mtree;