no_ofp: Use portId type synonym consistently

Some instances of VInt.t were used instead of portId. For more
documentation-like type signatures, use portId instead.
no_ofp: Use int64 for cost and capacity

Cost and capacity were using the VInt.t type event though they weren't
making use of any of its features. There was one exception where a
VInt.Int16 was constructed for the cost, that seemed unnecessary.
no_ofp: replace addr{MAC,IP} with Packet.{dl,nw}Addr

These types are already defined by the Packet module.
no_ofp: Use int64 for portId

Similar story to cost and capacity. OpenFlow 1.0 support 2^32 ports,
while OpenFlow 1.3 support 2^64 ports. Use the latter so the code works
with both.
no_ofp: Use int64 for switchId

The datapath_id is an int64 across OpenFlow standars, so just use that.
no_ofp: Remove ocaml-openflow as a dependency
Merge pull request #26 from frenetic-lang/no_ofp

Eliminate ocaml-openflow as a dependency