add avm2 exception handler support to assembler

add avm2 exception handler support to assembler

* mark beginning and end of active range with :%dlabel

* mark beginning of exception handler with
   (:%exception handler-name start-label end-label type-name variable-name)
  where type-name is name of type to catch, defaults to *
  and variable is name of slot for :new-catch using this exception block

* on entry to exception block, thrown object is on top of stack
  replacing any previous stack contents before the :throw

* (optional, i think?) create a catch object/scope with
  (:new-catch handler-name).  currently only works after corresponding
  :%exception instruction,  since it doesn't go back and resolve unknown
  exception labels after assembly

* somewhere within the dynamic extent of an exception block, push
  something on the stack, and call (:throw)