Enhancements to pa_goal:

 - Include the goal name, source file location in the generated
   rule so we can improve error messages.

 - Implement some sort of tracing based on the above.

It should be possible to register goal "atexit [of goal]" handlers.
These only run when the goal successfully exits, allowing:

 - implicit memory_set

 - implicit check_target

Multiple definitions of goals:

  let goal file_exists filename when filename `starts_with` "/foo" ->
    ...

  let goal file_exists filename when filename `starts_with` "/bar" ->
    ...

  - Might be implemented something like this:
    add_goal (`File_exists, fun patt -> raise Matches, fun patt -> expr)
    then when we run goals, we check them against all patterns (how?)
    and only if one matches to we run that function.  Multi-matches are
    not permitted.