0.9.15.36: less intrusive step instrumentation

 * INVOKE-EFFECTIVE-METHOD was missing a binding for the
   effective-method-form, causing potential multiple evaluation and
   also creating one source of confusion when step instrumenting CLOS
   code, manifesting as:

    Asserted type (MOD 536870911) conflicts with derived type
    (VALUES (OR FUNCTION SB-PCL::METHOD-CALL SB-PCL::FAST-METHOD-CALL)
            &OPTIONAL).

 * If the form being instrumented is a call to a known single-valued
   function we can instrument it in a way that doesn't kill the
   type-inference. This alone is enough to get rid of most warnings
   such as above.

 * Add rudimentary (B)acktrace command to the built-in stepper.