support executing code around rule transformations

* exported CALL-TRANSFORM: used by client code to invoke primary
  transform in :AROUND code

* added %around slot to class RULE

* added (:around (BOUNDS-SPEC) BODY) option to DEFRULE

* made COMPILE-RULE apply :AROUND code, if provided

* added simple symbol table example in example-symbol-table.lisp
simplify DEFRULE by using ALEXANDRIA:DESTRUCTURING-ECASE
turn test functions into EOS tests
fix typo, mention NOT in README