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...

lfs.h

@@ -268,6 +268,17 @@ struct lfs_config {
     // can track 8 blocks.
     lfs_size_t lookahead_size;
 
+    // Threshold for metadata compaction during gc in bytes. Metadata logs
+    // that exceed this threshold will be compacted during gc operations.
+    // Defaults to ~88% block_size when zero, though this default may change
+    // in the future.
+    //
+    // Note this only affects explicit gc operations. Otherwise metadata is
+    // only compacted when full.
+    //
+    // Set to -1 to disable metadata compaction during gc.
+    lfs_size_t gc_compact_thresh;
+
     // Optional statically allocated read buffer. Must be rcache_size. By
     // default lfs_malloc is used to allocate this buffer.
     void *rcache_buffer;