Added lfs_rbyd_rangesize (untested), some cleanup

Toying around with the idea that since rbyd trees have strict height
gaurantees after compaction (2*log2(n)+1), we can proactively calculate
the maximum on-disk space required for a worst case tree+leb128
encoding.

This would _greatly_ simplify things such as metadata compaction and
splitting, and allow unstorable file metadata (too many custom
attributes) to error early.

One issue is that this calculated worst case will likely be ~4-5x worst
than the actual encoding due to leb128 compression. Though this may be an
acceptable tradeoff for the simplification and more reliable behavior.

tests/test_rbyd.toml

@@ -1601,7 +1601,7 @@ code = '''
         printf("] ---\n");
 
         // build the attribute list for the current permutation
-        struct lfs_rattr attrs[N];
+        struct lfsr_attr attrs[N];
         for (unsigned j = 0; j < N; j++) {
             attrs[j] = *LFSR_ATTR2(
                     UATTR, perm[j]+1, -1,
@@ -2961,7 +2961,7 @@ code = '''
         printf("] ---\n");
 
         // build the attribute list for the current permutation
-        struct lfs_rattr attrs[N];
+        struct lfsr_attr attrs[N];
         for (unsigned j = 0; j < N; j++) {
             // adjust id based on future insertions
             uint16_t id = perm[j];