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Before, we envisioned exposing the following APIs:

	Prototype
	PrototypeNoSchema
	PrototypeOnlySchema
	Wrap
	WrapNoSchema

The names were long, though, and perhaps a bit confusing too. Instead,
just expose Prototype and Wrap, and allow either of the parameters to be
nil to infer its value. This makes the API easier to navigate, and we
can concentrate documentation in fewer places.

Now there's two main entrypoints: Prototype if one wants a high-level
prototype to create nodes of a certain shape, and Wrap if one wants to
expose an existing Go value as an IPLD node. Whether one supplies only
an IPLD Schema, only a Go type/value, or both, is their choice. The only
exception is Wrap, which always requires a Go value by definition.

Wrap works now, too, and gains an example. It's the same idea as the
previous Prototype example, but it builds the value via a Go composite
literal rather than fluent/qp on the IPLD side.

Another noteworthy change is that we now return TypedPrototype and
TypedNode rather than the regular NodePrototype and Node interfaces,
since bindnode always works with a schema - even when it's inferred.
The big advantage here is that one can call the Representation method
directly on the result without having to type assert, like the example
does.

Lots of TODOs and polishing to do, but it passes the schema/gen/go tests
with minimal changes.

Follow-up commits will continue filling in the gaps and adding
documentation, notably examples. We'll also make the tests run by
default, after a bit of refactoring.