minor cosmetic changes in esrap.lisp
mention "functions as terminals" in feature list in doc/esrap.texinfo
minor improvements in tests.lisp

* less redundancy in defrule.check-expression test

* consistent three-semicolon comments

* better variable names and assertion reports in around.{1,2} tests
better reports and tests for DEFRULE syntax errors

* use logical blocks in error condition reports

* new test DEFRULE.CONDITIONS tests signaling DEFRULE syntax errors