feat: compact encoding of graph edges

[m-hilgendorf]

Currently the encoding of graphs looks something like

tg.graph({
    "nodes": [
        {
            "kind": "file",
            "contents": tg.blob("foo.txt"),
            "dependencies": {
                "bar.txt": {
                    "index": 1,
                    "kind": "file"
                },
            }
        },
        {
            "kind": "file",
            "contents": tg.blob("bar.txt"),
            "dependencies": {
                "foo.txt": {
                    "index": 0,
                    "kind": "file"
                },
            }
        },
    ]
})

It is desirable to make this more compact for internal graph pointers where the kind field can be deduced from the rest of the node data, to encode this simply as

tg.graph({
    "nodes": [
        {
            "kind": "file",
            "contents": tg.blob("foo.txt"),
            "dependencies": {
                "bar.txt": 1
            }
        },
        {
            "kind": "file",
            "contents": tg.blob("bar.txt"),
            "dependencies": {
                "foo.txt": 0
            }
        },
    ]
})