Dropped lfsr_mptr_t as a struct

This replaces the lfsr_mptr_t struct with simple arrays.

The main motivation for this is C99's strict aliasing. It saves a
decent amount of stack to reference the mdir's internal block array as
an mptr directly, but we were only able to accomplish this in
lfsr_mdir_mptr by violating C99's strict aliasing rules.

The main downside of this is C's wonderful array-to-pointer decay
resulting in more implicit references and chances for things to get
clobbered (the original motivation for lfsr_mptr_t was due to bugs
introduced this way).

If I know one thing about C99's strict aliasing it's that it sure loves
to make code less safe.

No significant code changes, which is probably a good thing:

                     code          stack
  default before:   36436           2672
  default after:    36432 (-0.0%)   2672 (+0.0%)

  ckfetches before: 36674           2704
  ckfetches after:  36666 (-0.0%)   2704 (+0.0%)

lfs.h

@@ -536,10 +536,6 @@ typedef struct {
     lfs_size_t estimate;
 } lfsr_shrub_t;
 
-typedef struct lfsr_mptr {
-    lfs_block_t blocks[2];
-} lfsr_mptr_t;
-
 typedef struct lfsr_mdir {
     lfsr_smid_t mid;
     lfsr_rbyd_t rbyd;
@@ -706,7 +702,7 @@ typedef struct lfsr_traversal {
     union {
         // cycle detection state, only valid when traversing the mroot chain
         struct {
-            lfsr_mptr_t mptr;
+            lfs_block_t blocks[2];
             lfs_block_t step;
             uint8_t power;
         } mtortoise;
@@ -740,7 +736,7 @@ typedef struct lfsr_mtree {
         lfsr_mid_t weight;
         struct {
             lfsr_mid_t weight;
-            lfsr_mptr_t mptr;
+            lfs_block_t blocks[2];
         } mptr;
         lfsr_btree_t btree;
     } u;