types: Move ID output of StateElement

[lukechampine]

This is an experimental change that gives each element type an ID specific to that type, rather than all elements using types.Hash256 as their ID type. That is, sce.ID is now a types.SiacoinOutputID. This prevents awkward casts in a few places, namely when deriving output IDs like types.SiafundOutputID(sfi.Parent.ID).V2ClaimOutputID()) -- we can now write sfi.Parent.ID.V2ClaimOutputID() instead.

The second commit un-embeds the StateElement field from each element type. Embedding is best avoided unless there are significant benefits, and in this case the only significant benefit was convenient access to the ID field, which is now moot.

The combined effect on JSON encodings is:

old:
{
    "id": "h:9373b2995a6f5d47d63aaf22dbbfe92aa43b84551fe5de1e08341fd02dd100ec",
    "leafIndex": 12,
    "merkleProof": [
        "h:ee261e985aefd3c1f849310dc697b3b4c5d9227b48507d7d4399adefc90708ad",
        "h:f00470b20ca5da851e86ba6793bd967eac56bb7501b1189336b31384c7652d4a"
    ]
    "siacoinOutput": {
        "address": "111a37d4aedbbf671e6ffea289e1bcb8521ab9433fd2e26bce1432696943cbf26612a970021b",
        "value": 10000000000000000000
    },
    "maturityHeight": 0
}
new:
{
    "stateElement": {
        "leafIndex": 12,
        "merkleProof": [
            "h:ee261e985aefd3c1f849310dc697b3b4c5d9227b48507d7d4399adefc90708ad",
            "h:f00470b20ca5da851e86ba6793bd967eac56bb7501b1189336b31384c7652d4a"
        ]
    },
    "id": "scoid:9373b2995a6f5d47d63aaf22dbbfe92aa43b84551fe5de1e08341fd02dd100ec",
    "siacoinOutput": {
        "address": "111a37d4aedbbf671e6ffea289e1bcb8521ab9433fd2e26bce1432696943cbf26612a970021b",
        "value": 10000000000000000000
    },
    "maturityHeight": 0
}

This definitely breaks compatibility, so we'd have to weigh whether it's worth the annoyance.