Initial implementation of @LazyInject()

[guyca]

This PR introduces a @LazyInject decorator. The standard @Inject decorator resolves dependencies on class creation. The @LazyInject decorator resolves dependencies only when they are used for the first time.

TODO

• Implement @LazyInject
• Implement a new API to register graphs lazily:

  Obsidian.registerGraphs(graphs: Record<string, () => ObjectGraph>);

• Support passing an injection token to the @Injectable() decorator.
• Support passing an injection token to the subgraphs array property of the @Graph decorator.