regularise "given"

regularise "given"

Change "given" from implicitly enreferencing array and hash topics to just
giving the topic scalar context.  It's difficult to say whether this is a
change of the intended behaviour or merely a bugfix, because the implicit
enreferencement was not documented, and there were no tests exercising it.
The documentation merely said that the argument is in scalar context,
which correctly describes the new behaviour.  Add a documentation note
about the old behaviour, alongside the existing notes about historical
given/when behaviour.  Incidentally fix doc that referred to assiging
to $_, to instead refer to aliasing.  Add tests for "given".