Custom functions without using Internal modules

[tysonzero]

Came up in the context of #592

In the same way you declare a table and the types of the columns, seems fair to be able to declare a function and the types of the function args/result.

[tomjaguarpaw]

This seems reasonable. A nice API might provide a FunctionArgs product profunctor and support something like

makeFunction :: Default args args => String -> args -> FieldNullable n t

What do you think?

[tomjaguarpaw]

In fact, maybe FunctionArgs could just be Unpackspec. I'll have to check.