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.

[n8maninger]

This looks like a job for generics! /s

// A StateElement is a generic element within the state accumulator.
type StateElement[T ~[32]byte] struct {
	ID          T         `json:"id"` // SiacoinOutputID, FileContractID, etc.
	LeafIndex   uint64    `json:"leafIndex"`
	MerkleProof []Hash256 `json:"merkleProof,omitempty"`
}

[lukechampine]

does the encoding still work? Because honestly I think I'd be ok with that

[lukechampine]

Update: the encoding does work, but after fixing up 90% of the code, I hit this...

func (au ApplyUpdate) UpdateElementProof(e *types.StateElement) {
	au.eau.updateElementProof(e)
}

Go doesn't support generic methods, so we'd have to define a top-level function (ApplyElementProof?) for this, which sucks for various reasons (e.g. you can't define a ProofUpdater interface).

After reflecting on this some more, I think the approach taken in this PR is correct: ID should not be a field of StateElement. The ID itself actually has no bearing on the state accumulator whatsoever, except for its contribution to the leaf hash -- which is parameterized by type anyway.