Add persistence layer

[skiplabsdaniel]

The purpose of the persistence layer is to enable the Skip runtime to become the sole point of entry for data:

• Ensuring that only data in the Skip runtime is stored.
• Should speed up the loading of input values

A Store concept has been added, so that a service's "InitialData" elements can be an input array as it is today, or a Store (this allows mixing requirements, with different stores for different needs depending on the nature of the data, while still having the possibility of volatile data).
A store is a class that implements the interface:

export interface Store<K extends Json, V extends Json> {
  load(): Promise<Entry<K, V>[]>;
  save(data: Entry<K, V>[]): Promise<void>;
}

If the write in SKIP runtime fails the data is not stored, and if saving to a store fails, the corresponding write in the SKIP runtime will also fail.