rpc: Add "getburnreport" RPC function

[cyrossignol]

This adds an RPC function that aggregates the amounts of destroyed coins in confirmed transactions for reporting. It categorizes these amounts by contract type:

{
  "total": 3013.31027220,
  "voluntary": 259.78021420,
  "contracts": {
    "beacon": 1950.00000000,
    "message": 0.46905800,
    "poll": 201.02100000,
    "project": 21.00000000,
    "protocol": 2.00000000,
    "scraper": 4.00000000,
    "vote": 575.04000000
  }
}

[div72]

@cyrossignol Could you also use IsUnspendable in main.cpp#L795 and qt/transactionrecord.cpp#L48,329?

[cyrossignol]

@div72 These are all places where we want to look for OP_RETURN specifically.

• main.cpp#L795: The contract protocol requires an explicit OP_RETURN output. We don't want to allow other types of unspendables to be used for the burn fee.
• qt/transactionrecord.cpp#L48: This addresses an OP_RETURN-specific bug in old wallets.
• qt/transactionrecord.cpp#329: This hides the OP_RETURN output in the GUI on the sending side of a transaction that contains a custom message to avoid an extra line item for the contract.