Note [Eta reduction based on evaluation context]

Note [Eta reduction soundness], criterion (S) allows us to eta-reduce
`g (\x y. e x y)` to `g e` when we know that `g` always calls its parameter with
at least 2 arguments. So how do we read that off `g`'s demand signature?

Let's take the simple example of #21261, where `g` (actually, `f`) is defined as
  g c = c 1 2 + c 3 4
Then this is how the pieces are put together:

  * Demand analysis infers `<SC(S,C(1,L))>` for `g`'s demand signature

  * When the Simplifier next simplifies the argument in `g (\x y. e x y)`, it
    looks up the *evaluation context* of the argument in the form of the
    sub-demand `C(S,C(1,L))` and stores it in the 'SimplCont'.
    (Why does it drop the outer evaluation cardinality of the demand, `S`?
    Because it's irrelevant! When we simplify an expression, we do so under the
    assumption that it is currently under evaluation.)
    This sub-demand literally says "Whenever this expression is evaluated, it
    is called with at least two arguments, potentially multiple times".

  * Then the simplifier takes apart the lambda and simplifies the lambda group
    and then calls 'tryEtaReduce' when rebuilding the lambda, passing the
    evaluation context `C(S,C(1,L))` along. Then we simply peel off 2 call
    sub-demands `Cn` and see whether all of the n's (here: `S=C_1N` and
    `1=C_11`) were strict. And strict they are! Thus, it will eta-reduce
    `\x y. e x y` to `e`.