cosmetic changes
calloc should take size_t
rewrite pic_type_repr
fix logic flaw in GC
remove a field for debug of union header
set bitfield to union header.mark
GC_STRESS off by default
macroexpand_list supports any kind of improper list
exprs during macroexpansion may have syntactic-closure in the tail position
remove old debug prints
ir-macro-transformer was broken