(BLOCK_VARS, BLOCK_TYPE_TAGS, BLOCK_SUBBLOCKS, BLOCK_HANDLER_BLOCK):
Changed to reference fields of the new `struct tree_block' type.
(BLOCK_ABSTRACT_ORIGIN, BLOCK_ABSTRACT):  Definitions added.
(union tree_node):  Added `struct tree_block' as a new variant.

From-SVN: r1602