Make NamedSchema a data type

[fizruk]

To cite @jkarni:

if NamedSchema had an instance for HasParamSchema (which I guess means it shouldn't be a tuple) that would save me the chars . _2, for what it's worth

Having a separate type will also mean Name and Schema will be named fields rather than _1 and _2.

[jkarni]

I'm happy to do this, but NamedSchema would have to move (if we want the lenses for it to be in Lenses.hs) to avoid circular imports, and I'll defer to you on where to move it to (or whether to have the lenses elsewhere).