regularise "when"

regularise "when"

Remove from "when" the implicit enreferencement of array/hash conditions
and the implicit smartmatch of most conditions.  Delete most of the
documentation about behaviour of older versions of given/when, because
explaining the now-old "when" behaviour would be excessively cumbersome
and there's little compatibility to take advantage of.  Delete the
documentation about differences of given/when from the Perl 6 feature,
because the differences are now even more extensive and it's too much
difference to sensibly explain.  Add tests of "when" in isolation.