Pass a context (environment?) object to IAction.Execute() method

[dahlia]

Some operations such as random generation can depend on information determined by a mined block, that a transaction belongs to. However, since simply passing a Block object to IAction.Execute() may accidentally cause unpredictable dependencies in the future, the information should be limited to only what we actually need at present.

A type for a such information unit can be named Context or Environment, I think? @kijun @longfin Any suggestions are welcome.

IAction.Execute(Address, AddressStateMap)'s signature also should be changed like:

AddressStateMap Execute(Address from, Address to, AddressStateMap states, IContext ctx);

The IContext interface would look like at first:

public interface IContext {
    System.Random Random { get; }
}

[dahlia]

#55 implemented this. Thanks for @hanc1208.