Redesigned the inlined topology of files, now using geoxylic btrees

As a part of the general redesign of files, all files, not just small
files, can inline some data directly in the metadata log. Originally,
this was a single piece of inlined data or an inlined tree (shrub) that
effectively acted as an overlay over the block/btree data.

This is now changed so that when we have a block/btree, the root of the
btree is inlined. In effect making a full btree a sort of extended
shrub.

I'm currently calling this a "geoxylic btree", since that seems to be a
somewhat related botanical term. Geoxylic btrees have, at least on
paper, a number of benefits:

- There is a single lookup path instead of two, this simplifies code a
  bit and decreases lookup costs.

- One data structure instead of two also means lfsr_file_t requires
  less RAM, since all of the on-disk variants can go into one big union.
  Though I'm not sure this is very significant vs stack/buffer costs.

- The write path is much simpler and has less duplication (it was
  difficult to deduplicate the shrub/btree code because of how the
  shrub goes through the mdir).

  In this redesign, lfsr_btree_commit_ leaves root attrs uncommitted,
  allowing lfsr_bshrub_commit to finish the job via lfsr_mdir_commit.

- We don't need to maintain a shrub estimate, we just lazily evict trees
  during mdir compaction. This has a side-effect of allowing shrubs to
  temporarily grow larger than shrub_size before eviction.

  NOTE THIS (fundamentally?) DOESN'T WORK

- There is no awkwardly high overhead for small btrees. The btree root
  for two-block files should be able to comfortably fit in the shrub
  portion of the btree, for example.

- It may be possible to also make the mtree geoxylic, which should
  reduce storage overhead of small mtrees and make better use of the
  mroot.

All of this being said, things aren't working yet. Shrub eviction during
compaction runs into a problem with a single pcache -- how do we write
the new btrees without dropping the compaction pcache? We can't evict
btrees in a separate pass becauce their number is unbounded...

tests/test_btree.toml

@@ -4295,7 +4295,7 @@ code = '''
         assert(i <= 2*N);
 
         lfsr_binfo_t binfo;
-        int err = lfsr_btraversal_read(&lfs, &btree, &traversal, &binfo);
+        int err = lfsr_btree_traversalread(&lfs, &btree, &traversal, &binfo);
         assert(!err || err == LFS_ERR_NOENT);
         if (err == LFS_ERR_NOENT) {
             break;
@@ -4447,7 +4447,7 @@ code = '''
         assert(i <= 2*N);
 
         lfsr_binfo_t binfo;
-        int err = lfsr_btraversal_read(&lfs, &btree, &traversal, &binfo);
+        int err = lfsr_btree_traversalread(&lfs, &btree, &traversal, &binfo);
         assert(!err || err == LFS_ERR_NOENT);
         if (err == LFS_ERR_NOENT) {
             break;