This patch fixes Trac #6099 by reducing the result discount in CoreUnfold.conSize.
See Note [Constructor size and result discount] in CoreUnfold.

The existing version is definitely too aggressive. Simon M found it an
"unambiguous win" but it is definitely what led to the bloat. In a function
with a lot of case branches, all returning a constructor, the discount could
grow arbitrarily large.

I also had to increase the -funfolding-creation-threshold from 450 to 750,
otherwise some functions that should inline simply never get an unfolding.
(The massive result discount was allow the unfolding to appear before.)

The nofib results are these, picking a handful of outliers to show.

        Program           Size    Allocs   Runtime   Elapsed  TotalMem
--------------------------------------------------------------------------------
         fulsom          -0.5%     -1.6%     -2.8%     -2.6%    +31.1%
       maillist          -0.2%     -0.0%      0.09      0.09     -3.7%
         mandel          -0.4%     +6.6%      0.12      0.12     +0.0%
       nucleic2          -0.2%    +18.5%      0.11      0.11     +0.0%
        parstof          -0.4%     +4.0%      0.00      0.00     +0.0%
--------------------------------------------------------------------------------
            Min          -0.9%     -1.6%    -19.7%    -19.7%     -3.7%
            Max          +0.3%    +18.5%     +2.7%     +2.7%    +31.1%
 Geometric Mean          -0.3%     +0.4%     -3.0%     -3.0%     +0.2%

Turns out that nucleic2 has a function
  Main.$wabsolute_pos =
    \ (ww_s4oj :: Types.Tfo) (ww1_s4oo :: Types.FloatT)
      (ww2_s4op :: Types.FloatT) (ww3_s4oq :: Types.FloatT) ->
      case ww_s4oj
      of _
      { Types.Tfo a_a1sS b_a1sT c_a1sU d_a1sV e_a1sW f_a1sX g_a1sY h_a1sZ i_a1t0 tx_a1t1 ty_a1t2 tz_a1t3 ->
      (# case ww1_s4oo of _ { GHC.Types.F# x_a2sO ->
         case a_a1sS of _ { GHC.Types.F# y_a2sS ->
         case ww2_s4op of _ { GHC.Types.F# x1_X2y9 ->
         case d_a1sV of _ { GHC.Types.F# y1_X2yh ->
         case ww3_s4oq of _ { GHC.Types.F# x2_X2yj ->
         case g_a1sY of _ { GHC.Types.F# y2_X2yr ->
         case tx_a1t1 of _ { GHC.Types.F# y3_X2yn ->
         GHC.Types.F#
           (GHC.Prim.plusFloat#
              (GHC.Prim.plusFloat#
                 (GHC.Prim.plusFloat#
                    (GHC.Prim.timesFloat# x_a2sO y_a2sS)
                    (GHC.Prim.timesFloat# x1_X2y9 y1_X2yh))
                 (GHC.Prim.timesFloat# x2_X2yj y2_X2yr))
              y3_X2yn)
         } } }}}}},

        <similar>,
        <similar> )

This is pretty big, but inlining it does get rid of that F# allocation.
But we'll also get rid of it with deep CPR: Trac #2289. For now we just
accept the change.