Constants are like tiny programs, so they are bound by the same rules
as validators and other programs. In fact, functions are slightly more
flexible in that they allow generic constant expressions like
`List<a>`.
Yet, there is no way to contain such generic structure that contain
inhabitants in a way that satisfies the type-checker. In the example
of `List<a>`, the only inhabitant of that type that we can construct
is the empty list. Anything else would require holding onto some
generic value.
In addition, we can't force literal values into generic annotation, as
something like:
```
const foo: List<a> = [1, 2, 3]
```
wouldn't type-check either since the right-side would unify to
`List<Int>`. And again, the only right-hand side that can type-check
is the empty list without any inhabitant.
The added restriction on generic function is necessary because while
we allow constants to return lambda, we cannot (easily) generate UPLC
that is generic in its argument. By the time we generate UPLC, the
underlying types have to be known.
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