Implement p4-to-bpf translation, a BPF interpreter, and flow extraction.

The p4-to-bpf translation here is only good enough to implement pretty much
exactly what ovs.p4 needs.  I only tested that the following command yields
correct-looking results:

tests/ovstest test-p4 flow-extract ../tests/ovs.p4 ffffffffffff000102030405810092345678

Open vSwitch version 1.10.2.

Open vSwitch version 1.9.3.