Implement ReceiptView's Receipt parsing

[rqzrqh]

type ReceiptView struct {
PredecessorID types.AccountID json:"predecessor_id"
ReceiverID types.AccountID json:"receiver_id"
ReceiptID hash.CryptoHash json:"receipt_id"
Receipt json.RawMessage json:"receipt" // TODO: needs a type!
}

Now the type of the Receipt field is RawMessage, I have to parse it by gjson.
Of course the better way is use a Receipt Struct.
In nearcore the receipt is a Enum Type, it seems a bit hard to unmarshal it.

[mikroskeem]

Enum parsing is not going to be very pretty, see e.g Action struct. I'll see during this week, but PRs are accepted in the meantime.