Flux.from() is cheap and can be used to marshal the inputs everywhere
internally. With this change users ought to be able to register any
function of any Publisher type.
Makes it possible to support other "function" types in the future.
The user is always taking a risk with the lookup that the object
returned has the generic type desired (but that hasn't changed
with this commit). FunctionCatalog is a lot simpler as a result
and also a lot more flexible.
