flow: Remove flow_to/from_match() in favor of cls_rule_to/from_match().

flow: Remove flow_to/from_match() in favor of cls_rule_to/from_match().

The flow_from_match() and flow_to_match() functions have to deal with most
of the state in a cls_rule anyhow, and this will increase in upcoming
commits, to the point that we might as well just use a cls_rule anyhow.
This commit therefore deletes flow_from_match() and flow_to_match(),
integrating their code into cls_rule_from_match() and the new function
cls_rule_to_match(), respectively.  It also changes each of the functions'
callers to use the new cls_rule_*() function.