Document strict fields invariant

Suppose MkT :: !a -> T a.

As far as I understand, strict fields are only a syntactic sugar for case expressions in Core wrappers. In Core, it is possible form MkT undefined, bypassing wrappers. While we don't expose workers directly in Haskell, there's no robust guarantee that will never happen. Wrappers are inlined; perhaps some kind of nefarious rewrite rule, an optimisation or a plugin could form the application of a worker MkT x without forcing x. This will pass Core Lint.

GHC relies on the fact that MkT does not contain a thunk. Note [Adding evaluatedness info to pattern-bound variables] means that we remove evaluation of fields known to be strict. I've also heard that the code generator can exploit strictness of fields, but I could not confirm. In this case things could really go wrong if there's an unexpected thunk.

This ticket is a proposal: let's add to the list of Core invariants that a datacon worker with a strict field must be called with an evaluated argument.

I guess a robust fix would be to track strictness in types as in the "Types are calling conventions" paper, but that's expensive.