… kinds

also adds tests, and typed functions that must be used in the interface

Can handle maps, lists and all scalar kinds; a receiving Go type that is
registered as a CustomTypeAnyConverter will receive a datamodel.Node and is
expected to return one when converting to and from that custom Go type.

API docs are too verbose

For centralising bindnode setup - register a type & schema once and not have to
worry about the TypedPrototype or direct bindnode calls after that. It also
caches bindnode Options that may go along with the type.

This could have been a global registry, but there exists a possibility of the
same type wanting to be used by different users in the same application
instance but with different schemas and/or Options. So instead, the question of
it being a global is left to the user.