Annotate NamedSchema with used Definitions - #19

In the process of making a list of schema definitions generically I have discovered a (previously unknown?) Declare monad. Although I think we can use a lazy State monad here with declare = modify . (<>), I find it appealing that Declare by nature disallows removing prior definitions.

This code is already working (see tests here).

I am going to clean up the code before the merge.