Add .foreach() and .foreach_body() iterators

This MR adds two new methods on regex objects to iterate over all matches in a string or body by calling a VCL subroutine each match:

  BOOL <obj>.foreach(STRING, SUB, [, INT limit] [, INT limit_recursion])
  BOOL <obj>.foreach_body(req_body | bereq_body | resp_body, SUB,
                          [, INT limit] [, INT limit_recursion])

Relevant bits of examples to illustrate:

• collect all matches from req.http.input in resp.http.all:

	sub myregex_collect {
		set resp.http.all += myregex.backref(0);
	}

	sub vcl_synth {
		unset resp.http.all;
		myregex.foreach(req.http.input, myregex_collect);
	}

• collect two a and/or b keys with their values from & separated pairs in the request body:

	sub vcl_init {
		new pattern = re.regex("(?:^|&)(a|b)=([^&]*).*?&(a|b)=([^&]*)",
		    forbody=true);
	}

	sub collect {
		set resp.http.all +=
		    pattern.backref(1) + "=" + pattern.backref(2) + "," +
		    pattern.backref(3) + "=" + pattern.backref(4) + ";";
	}

	sub vcl_synth {
		unset resp.http.all;
		if (pattern.foreach_body(req_body, collect)) {
			set resp.status = 200;
		}
		return (deliver);
	}

	sub vcl_recv {
		return (synth(400));
	}

Note: The particular examples are super simple and the respective functionality could be achieved more efficiently by other means. The intention here is solely to provide (hopefully) easy to understand illustrations.