t: (Re)implemented gc_compact_thresh, at least over mdirs

lfs_fs_gc is still not reimplemented, but this is accessible through the
traversal API with LFS_T_COMPACT.

This is also the first traversal operation that can mutate the
filesystem, which brings its own set of problems:

- We need to set LFS_F_DIRTY in lfsr_mtree_gc now, which really
  highlights how much of a mess having two flag fields is...

  We do _not_ clobber in this case, since we assume lfsr_mtree_gc knows
  what it's doing.

- We can now commit to an mroot in the mroot chain outside of the normal
  mroot chain update logic.

  This is a bit scary, but should just work.

  The only issue so far is that we need to allow mdirs to follow the
  mroot during mroot splits if mid=-1, even if they aren't lfs_t's mroot
  mdir.

  This should now be decently tested with the new
  test_traversal_compact_* tests.

- It's easy for mtraversal's mdir and mtinfo's mdir to fall out of sync
  when mutating... Why do we have two of these?

The actual compaction itself is pretty straightforward: just mark as
unerased, eoff=-1, and call lfsr_mdir_commit with an empty commit. This
is now wrapped up in lfsr_mdir_compact.

Code changes:

           code          stack
  before: 34528           2640
  after:  34652 (+0.4%)   2640 (+0.0%)

Though the real hard part will be implementing gc_compact_thresh over
btree nodes...

runners/test_runner.h

@@ -105,6 +105,7 @@ void test_permutation(size_t i, uint32_t *buffer, size_t size);
     TEST_DEFINE(PCACHE_SIZE,        LFS_MAX(16, PROG_SIZE)                  ) \
     TEST_DEFINE(FILE_BUFFER_SIZE,   16                                      ) \
     TEST_DEFINE(LOOKAHEAD_SIZE,     16                                      ) \
+    TEST_DEFINE(GC_COMPACT_THRESH,  0                                       ) \
     TEST_DEFINE(INLINE_SIZE,        BLOCK_SIZE/4                            ) \
     TEST_DEFINE(SHRUB_SIZE,         INLINE_SIZE                             ) \
     TEST_DEFINE(FRAGMENT_SIZE,      BLOCK_SIZE/8                            ) \
@@ -134,6 +135,7 @@ void test_permutation(size_t i, uint32_t *buffer, size_t size);
     .pcache_size        = PCACHE_SIZE,          \
     .file_buffer_size   = FILE_BUFFER_SIZE,     \
     .lookahead_size     = LOOKAHEAD_SIZE,       \
+    .gc_compact_thresh  = GC_COMPACT_THRESH,    \
     .inline_size        = INLINE_SIZE,          \
     .shrub_size         = SHRUB_SIZE,           \
     .fragment_size      = FRAGMENT_SIZE,        \