WorkWrap: Deprecate `-ffun-to-thunk` in favor of full laziness
In #21192, I dicovered that `-ffun-to-thunk` is basically useless, because full laziness will float out a thunk anyway. E.g., I tried the following program:
```haskell
f :: Int -> Int
f x = sum (g x) + sum [0..x] + sum (g (x+2))
where
g _ = [0..x]
{-# NOINLINE g #-}
```
and even without `-ffun-to-thunk`, we get
```
$wf
= \ ww_s1In ->
let { lvl_s1J6 = eftInt 0# ww_s1In } in
let { $wg_s1HW = \ @ p_s1HU (_ :: Void#) -> lvl_s1J6 } in
...
```
So even though we don't make `$wg` into a thunk, we still float out a thunk out of `$wg`'s constant body. So it appears `-ffun-to-thunk` doesn't have any effect after all if `-ffull-laziness` is on. To put it another way:
| ftt | full laziness | do we get a thunk? |
|-----|----------------|-------------------|
| on | on | yes |
| off | on | yes |
| on | off | yes |
| off | off | no |
As you can see, `-ffun-to-thunk` only makes a difference if full laziness is off. Who deactivates full laziness but wants WW to still create thunks by passing `-ffun-to-thunk`?!
Therefore, I propose that `-ffun-to-thunk` is always off and the flag deprecated.
The advantages are:
1. One flag less (to explain)
2. `-ffull-laziness` governs whether or not we will float out a thunk
3. We get to delete the code below
Code below:
```
-- If fun_to_thunk is False we always keep at least one value
-- argument: see Note [Protecting the last value argument]
-- If it is True, we only need to keep a value argument if
-- the result type is (or might be) unlifted, in which case
-- dropping the last arg would mean we wrongly used call-by-value
needs_a_value_lambda
= not fun_to_thunk
|| might_be_unlifted
-- Might the result be lifted?
-- False => definitely lifted
-- True => might be unlifted
-- We may encounter a representation-polymorphic result, in which case we
-- conservatively assume that we have laziness that needs
-- preservation. See #15186.
might_be_unlifted = case isLiftedType_maybe res_ty of
Just lifted -> not lifted
Nothing -> True
```
Since `not fun_to_thunk` and thus `needs_a_value_lambda` will always be true, we can delete all this.
issue