Export utility types

[teunmooij]

Fixes #20

Exports some helper types from the module. These helpers are very useful in some edge cases, but in most situations usage is discouraged:

• RegistrationsOf can be used when working with subsystems, but it's usually better to just specify the required dependencies on the main system explicitly. Over use of this type can use to Infinite debt issues.
• TypeOf can be used to get the type of the built system, but we should encourage the use of async/await patterns rather than callbacks and in such situations we usually don't need to specify the type.

Since the types are easy to find when you need them, I did not add them to the documentation.

• Tests pass
• [] Appropriate changes to README are included in PR