Allow to create singleton services

[nfroidure]

A singleton service instance is available across multiple running silos. It is maintained in a WeakMap in order to avoid spawning multiple instances of a service while ensuring unused services can still be garbage collected.

import { depends, service } from './knifecycle';
import LoggerService from 'logger';

service('logger', depends('stdout', LoggerService), { singleton: true });

Consider checking singleton dependencies are also singletons, not sure it is that important, but with no use case for singleton depending on non-singleton, it may help prevent programming errors.