Extracted filter function from `separate-filters`

I hope I correctly understood you TODO.

* src/midje/parsing/other/arglists.clj:
  - Moved filter function from `separate-filters` to
  `desired-fact-predicate-from`.
* src/midje/repl.clj:
  - Updated `let` bindings to use the new function.
* test/midje/parsing/other/t_arglists.clj:
  - Updated tests