Add SBCL-style trace method for method combinations

[camsaul]

SBCL lets you trace method combinations:

(defgeneric foo (x)
  (:method (x) 3))
(defmethod foo :around ((x fixnum))
  (1+ (call-next-method)))
(defmethod foo ((x integer))
  (* 2 (call-next-method)))
(defmethod foo ((x float))
  (* 3 (call-next-method)))
(defmethod foo :before ((x single-float))
  'single)
(defmethod foo :after ((x double-float))
 'double)

(trace foo :methods t)

(foo 2.0d0)
  0: (FOO 2.0d0)
    1: ((SB-PCL::COMBINED-METHOD FOO) 2.0d0)
      2: ((METHOD FOO (FLOAT)) 2.0d0)
        3: ((METHOD FOO (T)) 2.0d0)
        3: (METHOD FOO (T)) returned 3
      2: (METHOD FOO (FLOAT)) returned 9
      2: ((METHOD FOO :AFTER (DOUBLE-FLOAT)) 2.0d0)
      2: (METHOD FOO :AFTER (DOUBLE-FLOAT)) returned DOUBLE
    1: (SB-PCL::COMBINED-METHOD FOO) returned 9
  0: FOO returned 9
9

We should add debugging facilities to Methodical to add the same functionality