Automatically decode XDR structures for Server.getEvents

[Shaptic]

This makes the following changes to the expected schema for getEvents endpoint:

-   contractId: string;
+   contractId: Contract;
-   topic: string[];
+   topic: xdr.ScVal[];
-   value: {
-     xdr: string;
-   };
+   value: xdr.ScVal;

In particular,

• events[i].contractId is now an instance of Contract
• events[i].topic is now a list of decoded xdr.ScVal instances
• events[i].value.xdr is now remapped directly to events.value
• events[i].value is a decoded xdr.ScVal instance

Related: #128.