Make memory extensible

[danielberndt]

This is an updated version of #107 with just the minimal changes.

Once this PR is in, it'll be very easy to extend your object's memory in your own index.d.ts file like this:

interface Room {
  memory: RoomMemory;
}

interface RoomMemory {
  spawnerId?: string;
}

[thaelina]

Gets rid of some unnecessary any references and makes Memory extensible.