Formalize the notion of `#[fundamental]`

[Centril]

Context: #703

I think we sorta want to give a description of behavior for any arbitrary fundamental type that is semi-syntactic in that it talks about type arguments and type constructors and how those relate wrt. locality and such. Then we can itemize a list of fundamental type constructors.

I changed the fundamental types part to use the type constructor wording, but I leave it to Niko to define what they are :)

cc @nikomatsakis