eqv? not a generic, equal? dispatches to generic only for objects

eqv? not a generic, equal? dispatches to generic only for objects

* libguile/eq.c (scm_eqv_p): Not a generic any more. Since eqv? is used
  by e.g. `case', which should be able to compile into dispatch tables,
  it really doesn't make sense to dispatch out to a generic.
  (scm_equal_p): So it was always the case that (equal? 'foo "foo") =>
  #f. But (equal? 'foo 'bar) could actually be extended by a generic.
  This was a bug, if you follow the other logic of the code. Changed so
  that generic functions can only extend the domain of equal? when
  operating on goops objects.

* oop/goops.scm: No more eqv? generic.

* test-suite/tests/goops.test: Remove eqv? tests.