added docstring to tick-robot
wrote init-world function to create a world-state
changed name of gosub-test
got rid of the special 'nth-state' helper function in exec-test.clj
got simple printing of world almost working
altered informal testing junk in core.clj
changed name 'compile' to 'assemble', now don't have to exclude loading of clojure.core/compile
rearranged indexing of registers
refactored exec-test and wrote new test for pushing and pulling from RANDOM register
added test for INDEX/DATA pair of registers
minor change: for changing equality, moved from set notation to equals sign
moved tokens' :line and :pos fields into the metadata map, refactored to preserve metadata, wrote a test for that
revised informal testing junk in core.clj