(WIP) Expose registry API for third-party integrations

[brentyi]

Motivated by #23, just starting a PR to track progress.

The current goal is to:

1. Expose an API for configuring custom behavior/constructors for types that match some criteria. This is particularly useful for protocols, which can only be instantiated by an external function.
2. Prove that this API is powerful enough by internally migrating support for dataclasses, attrs, TypedDict, etc, over to it.

Turns out, however, that the engineering complexity of (2) is pretty high when we start considering all of the corner cases concerning things like partials, generics, helptext generation, forward references, etc, that the existing architecture was designed to handle.

Given time constraints, may need to choose between (a) not landing this feature for the forseeable future or (b) reverting a bunch of changes and making the PR less ambitious.