oxm_fields: add some routines

these will be used by the later OFPMatch and OFPActionSetField changes.
of12: make OFPMatch use oxm_fields routines
of12: change OFPMatch json representation to match with on-wire

old:
    { "OFPMatch": {
        "eth_dst": "f2:0b:a4:7d:f8:ea"
    }}

new:
    { "OFPMatch": {
        "oxm_fields": [
            { "OXMTlv": {
                "field": "eth_dst",
                "mask": null,
                "value": "f2:0b:a4:7d:f8:ea"
            }}
        ]
    }}
of12: update *.json after OFPMatch changes
of13: make OFPMatch use oxm_fields routines
of13: change OFPMatch json representation to match with on-wire

old:
    { "OFPMatch": {
        "eth_dst": "f2:0b:a4:7d:f8:ea"
    }}

new:
    { "OFPMatch": {
        "oxm_fields": [
            { "OXMTlv": {
                "field": "eth_dst",
                "mask": null,
                "value": "f2:0b:a4:7d:f8:ea"
            }}
        ]
    }}
of13: update *.json after OFPMatch changes