hask-key build process to look like a function call instead:
	hash_data(req.http.foo_bar) 
and allow string concatenation while we're at it.

the name "hash_data()" is not quite to my liking, but I want to avoid
"hash()" since that clashes with previous syntax for "return(hash)".



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5001 d4fa192b-c00b-0410-8231-f00ffab90ce4