Reworked gstate/commit interactions

The main change is moving away from applying gstate changes via special
attrs. Instead, gstate changes are applied implicitly, whenever the
relevant field in lfs_t differs from the gstate on-disk.

How do we recover from errors then? Well, we already need to track the
exact on-disk encoding of any gstate (grm_p) to avoid issues with minor
encoding differences, so if we encounter an error, we can revert any
changes to gstate by re-decoding the on-disk gstate. This is more
fragile: 1. all error paths in lfsr_mdir_commit need to revert gstate,
2. logic must not error between gstate updates and lfsr_mdir_commit, but
it gets the job done.

The benefit of this approach is that it's much easier to manipulate
gstate inside of lfsr_mdir_commit. No more hacky attr-list scanning to
patch grms mid-commit! It also in theory saves stack usage by dropping
an attr, but none of these attrs were on our stack hot-path.

Other gstate changes:

- Moved all grm adjustments into lfsr_mdir_commit.

  This should deduplicate the messy grm adjust logic and make grms
  easier to work with.

  One hiccup though is the temporarily self-removing bookmark created in
  lfsr_mkdir, which needs to create a grm referencing an mid that
  doesn't exist yet. To work around this, lfsr_mdir_commit now
  automatically creates grms for new bookmarks.

  This might be a problem if we ever elide same-mdir mkdirs, but if so
  we can solve that problem then.

- Dropped lfsr_data_t xoring, the added complexity wasn't really worth
  it since all gstate should be small enough to buffer on the stack.

- Renamed several things:
  - lfsr_grm_push/poprm -> lfsr_grm_push/pop
  - lfsr_grm_isrm -> lfsr_grm_ispending
  - grm_g -> grm_p
  - grm.rms -> grm.mids

- Moved things around so grm/gstate logic is grouped together.

Unfortunately none of these attrs were on our stack hot-path, so no
stack savings. But thanks to the simpler logic, this does save quite a
bit of code:

           code          stack
  before: 33514           2632
  after:  33338 (+0.5%)   2640 (+0.3%)

lfs.h

@@ -568,7 +568,7 @@ typedef struct lfsr_mtree {
 #define LFSR_GRM_DSIZE (1+5+5)
 
 typedef struct lfsr_grm {
-    lfsr_smid_t rms[2];
+    lfsr_smid_t mids[2];
 } lfsr_grm_t;
 
 // The littlefs filesystem type
@@ -614,7 +614,7 @@ typedef struct lfs {
     } lookahead;
 
     lfsr_grm_t grm;
-    uint8_t grm_g[LFSR_GRM_DSIZE];
+    uint8_t grm_p[LFSR_GRM_DSIZE];
     uint8_t grm_d[LFSR_GRM_DSIZE];
 } lfs_t;