lc-sig: add functions related to semantics

Provide a way to apply an action to a packet, as well as various ways to
check patterns against header-map-like things.
lc-sig: add packet-related quickcheck instances

Note that this commit uses interfaces exposed by the openflow.quickcheck
module. As of this commit those interfaces had not yet been merged.
lc-sig: add quickcheck test for seq_pat_act

Currently requires that the Pattern.t type not be abstract.