Fixed a couple of issues with bignums. Most notably, negative_carry
treea02d7cf91c56aceff6263fc253a6f48d9588397e
-rw-r--r-- 10769 library.scm
-rw-r--r-- 16549 math.scm
-rw-r--r-- 89166 picobit-vm.c
-rw-r--r-- 112041 picobit.scm
-rw-r--r-- 31589 robot.scm