Provide access to container._registry

[leppaott]

Hello, would be nice if there was an easy way to access container._registry for iterating the token-registration pairs.

My use case currently is to iterate the instances to execute some async operations without me keeping track of which instances are really used. So I'm doing what this code does basically https://github.com/microsoft/tsyringe/blob/master/src/dependency-container.ts#L400