Pass bindings into imported modules

[AshleyYakeley]

Something like:

given
    x : T;
in
let
    y = f x;
end

on interpret:

• require T ambipolar (or just negative?)
• create new varid v
• create entry for x of v with type T

on import:

• lookup x to get expr
• substitute expr for v in y

[AshleyYakeley]

Doing #255 instead.