rpc: Add 'explainrawtransaction' RPC to decode a raw tx with fees, etc.

When decoding raw transactions, it is useful to have information about
the inputs beyond their txid and vout (if this information is available).
This patch adds an RPC analogous to decoderawtransaction, which looks up
the inputs of the raw transaction.

It outputs the following information beyond decoderawtransaction:
  o txout data on all the inputs (everything from gettxout)
  o Total input and output values, and the implied fee
  o the priority of the transaction

Since all this information depends on the inputs being available, if the
utxo lookup fails for any input, the RPC fails, telling the user to use
decoderawtransaction instead.

The JSON output is also contained in a list, unlike decoderawtransaction.
A future extension will be to allow the user to explain several (linked)
transactions in one go, using outputs from earlier transactions when
looking up inputs for later ones. (This is not implemented.)