Made grm a special attribute, moved encoding into mdir commit

This is entirely a pragmatic change, lfsr_mdir_commit already does
several hairy things with grm tags, decoding, fixing, reencoding, etc,
so it makes sense to move all the encoding logic into lfsr_mdir_commit.

This leads to a couple optimizations:

- We don't need to decode the grm to apply any last minute fixes.

- By allowing the grm arugment to be mutated (they are just sitting on
  the stack anyways, we need a copy in case we back out of change due to
  error), we can apply and save any grm fixes in the grm argument
  itself.

  This means we only need to fix the grm at most once, after any mtree
  modifications.

Which in turn saves some code and stack cost:

            code          stack
  before:  22930           2392
  after:   22706 (-1.0%)   2344 (-2.0%)

lfs.h

@@ -499,11 +499,9 @@ typedef struct lfs {
     lfsr_mdir_t mroot;
     lfsr_btree_t mtree;
 
-    // TODO do we really need separate decoded/encoded grms?
-    lfsr_grm_t grm_;
-
-    uint8_t grm[LFSR_GRM_DSIZE];
-    uint8_t grmd[LFSR_GRM_DSIZE];
+    lfsr_grm_t grm;
+    uint8_t pgrm[LFSR_GRM_DSIZE];
+    uint8_t dgrm[LFSR_GRM_DSIZE];
 
     // linked-lists of opened mdirs, we keep a separate linked-list
     // for each type since these need to be handled a bit differently