Cleanup, fixed inconsistent names, moved btree attr allocation up

- len => size - these all refer to byte-arrays
- buf => buffer - this doesn't matter but buffer is currently used more
- delta => d - we use delta for weight deltas, gstate deltas, using a
  slightly different name (if somehow even less descriptive) for byte
  offset-offsets helps avoid name collisions a little bit

The storage changes in btree operations should've probably been a
separate commit but got wrapped up in these changes. Now the high-level
btree operations are responsible to the attr storage for all internal
btree commits, as defined by LFSR_BTREE_SCRATCHATTRS.

This leads to slightly less total RAM usage, since it allows the
low-level btree operations to cannibilize the attrs of the high-level
btree operations as a part of its unrolled-tail-recursive
implementation.

This also includes some other cleanup such as removing old commented out
parts.

lfs.h

@@ -373,8 +373,8 @@ typedef union lfsr_btree {
     struct {
         lfs_ssize_t weight;
         lfsr_tag_t tag;
-        uint16_t len;
-        uint8_t buf[LFSR_BTREE_INLINE_SIZE];
+        uint16_t size;
+        uint8_t buffer[LFSR_BTREE_INLINE_SIZE];
     } inlined;
 } lfsr_btree_t;
 
@@ -391,7 +391,7 @@ typedef union lfsr_btree {
 //    union {
 //        struct {
 //            uint8_t size;
-//            uint8_t buf[LFSR_BTREE_INLINE_SIZE];
+//            uint8_t buffer[LFSR_BTREE_INLINE_SIZE];
 //        } inlined;
 //
 //        // if we're not inlined, point to the trunk rbyd block of the btree