This implements a sort of "safety net", adding some compile and runtime
error checking and reporting.


The first of these is to extend the `@` and `!` prefixes to check for
the existence of a word name. In a basic image, if the name isn't found,
it will resolve to address 0 silently. This will have it report an error
if the word is not found.

~~~
:err:var-not-defined
  '\nERROR:_variable_%s_not_defined\n s:format s:put bye ;

:if:not-defined (sq-s)
  over d:lookup n:zero? swap if ;

:prefix:@
  [ err:var-not-defined ] if:not-defined
  d:lookup d:xt fetch class:data |fetch ; immediate

:prefix:!
  [ err:var-not-defined ] if:not-defined
  d:lookup d:xt fetch class:data |store ; immediate
~~~