erts: store AVL balance in low bits of ordered_set node left pointer

Encode the AVL balance factor in the two low bits of the TreeDbTerm left
pointer instead of a separate field, reducing memory per node on 64-bit
(one word saved per node). Requires 8-byte-aligned nodes
(ERTS_ALLOC_ALIGN_BYTES >= 8).

The linux kernel implements a similar trick for its Red-Black trees,
storing the bits for the colour at the lowest bits of the pointer to the
parent.

Author: NelsonVides