Improve stringification of captor.capture() matcher

right now, captor.capture() matchers are printed out with an ugly toString() in error messages. they should be niceified like the other matchers.

currently just got:

All calls of the test double, in order were:
  - called with `("target", undefined, function (name, originalValue) { 
      return (0, _chainPrototype2.default)(overwriteChild(originalValue, '.' + name), name, originalValue, original); 
    })`.                                                                                    

That whole third arg should have been captor.capture() or the like

Start by locating the captor.capture() matcher and the stringification path used for matcher error messages, then compare it with the formatting used by other matchers. Reproduce the shown call-output case and verify that the third argument is rendered as captor.capture() or equivalent rather than the generated function source.