Adopted lfsr_data_from* pattern for internal data encoding

Taking advantage of the fact that these functions should never error,
changing the return type to lfsr_data_t allows all of the encoding
information to be passed around quite easily.

And, by giving each lfsr_data_from* function an LFSR_DATA_FROM* macro,
these functions can participate in our attr-list generating macros:

  LFSR_ATTR(-1, MTREE, 0, FROMBTREE(lfs, mtree, mtree_buf))

Though one thing to watch out for is the borrowed buffer that stores the
actual data. This might welcome use-after-free bugs since it's not super
clear the buffer remains borrowed. Will need to watch out for this.

tests/test_mtree.toml

@@ -4243,11 +4243,11 @@ code = '''
     lfsr_mount(&lfs, CFG) => 0;
     lfs_alloc_ack(&lfs);
 
-    uint8_t buffer[LFSR_MDIR_DSIZE];
-    lfs_ssize_t d = lfsr_mblocks_todisk(&lfs, LFSR_MBLOCKS_MROOTANCHOR, buffer);
-    assert(d >= 0);
+    uint8_t buf[LFSR_MDIR_DSIZE];
     lfsr_mdir_commit(&lfs, &lfs.mroot, LFSR_ATTRS(
-            LFSR_ATTR(-1, MROOT, 0, BUF(buffer, d)))) => 0;
+            LFSR_ATTR(-1,
+                MROOT, 0, FROMMBLOCKS(&lfs, LFSR_MBLOCKS_MROOTANCHOR,
+                    buf)))) => 0;
 
     // technically, cycle detection only needs to work when we're validating
     lfsr_mtree_traversal_t traversal = LFSR_MTREE_TRAVERSAL(