`c-expr-dsl`: Split type language into types of types and types of values

[dschrempf]

This is relevant only after we merge well-typed/hs-bindgen#1862, which introduces a type for types MacroTypeTyCon into c-expr-dsl.

Having MacroTypeTyCon as part of DataTyCon is minimally annoying, because we have to panic when we encounter the type of types when analyzing the type of a value expression.

See the relevant TODO items in the code.