Patch for Benchmark.pm

Patch for Benchmark.pm

This patch is a cleanup for the Benchmark module. It documents all the
stuff that wasn't, including my recent patch to allow code references;
it removes the old perl-comment docs that duplicate the POD; it fixes
some minor bugs (mostly of the '$arg ||= $default' instead of
'$arg = $default unless defined $arg' variety) and generally tidies
things up a bit.

Let me know if any of this causes problems,

p5p-msgid: 199703291504.PAA01596@crypt.compulink.co.uk
Signed-off-by: Hugo van der Sanden <hv@crypt.compulink.co.uk>