Reverted non-dag file bshrubs/btrees

Ok so, funny story, looks like we won't actually need pure-tree
bshrubs/btrees.

It _is_ true that the single-parent constraint imposed by pure-trees can
enable a wider range of algorithms. But looking forward into the planned
design, we just happen to not need this constraint at all. I made a
mistake here:

1. Block allocation - On paper block allocation benefits the most from
   the single-parent constraint. But we have another daggish problem,
   how do we efficiently account for in-flight/open btrees?

   Naively, you might think we can just traverse all open btrees during
   allocation, since we shouldn't have _that_ many. But this scales
   O(n^2) when writing a large file. The key observation being that open
   files reference on-disk btrees and are _not_ RAM constrained.

   The current solution involves tree-diffing in order to figure out
   bmap updates. Which, humorously, works perfectly fine even if the
   trees are dags.

2. Error correction - I just completely forgot that the current plans
   for block redundancy require the ddtree.

   Each block gets mapped into the dense ddtree, with subranges of the
   ddtree grouped into parity groups backed by the ptree. Instead of
   bptrs, file btrees store indirect ddkeys into the ddtree. No bptrs?
   No dag problem!

   This is still a problem if we ever support naive data redund (redund
   blocks in a bptrs), but that's out of scope for other reasons
   (basically just a lot more code).

So reverting. Allowing dags allows for much faster random writes, at
least in theory.

---

For now I'm still keeping the dag-avoidance in lfsr_file_flush_ around
under the LFS_NONDAG ifdef. This will likely be dropped at some point,
but I'm curious how it affects benchmarks.

Ugh, and of course the unused label makes GCC unhappy. Added
-Wno-unused-label to CFLAGS because labels have other uses besides just
being goto targets (debug targets, code organization, etc).

We probably use labels more that other libraries because to littlefs's
no-recursion requirement.

Code changes minimal, still not sure where that stack difference comes
from:

           code          stack          ctx
  before: 35740           2424          640
  after:  35736 (-0.0%)   2440 (+0.7%)  640 (+0.0%)

Makefile

@@ -74,6 +74,8 @@ CFLAGS += -fcallgraph-info=su
 CFLAGS += -g3
 CFLAGS += -I.
 CFLAGS += -std=c99 -Wall -Wextra -pedantic
+# labels are useful for debugging, in-function organization, etc
+CFLAGS += -Wno-unused-label
 # compiler bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101854
 CFLAGS += -Wno-stringop-overflow
 CFLAGS += -ftrack-macro-expansion=0