feat: Introduce composable type errors and type parameter checking

This refactoring moves the type definitions into a recursively
evaluated attribute set, which makes it possibe to refer to types in
the definitions of other types.

This functionality is used to type-check some of the type parameters
passed in to other types. This is not yet done for all higher-order
types because the evaluation semantics of Nix make it less useful for
some cases.

Further the definition of a type itself is now a stricter
representation that works for all types except typed functions, which
allows for composable error messages.

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