size_expr now ignores RealWorld lambdas, arguments, and applications.

Worker-wrapper previously removed all lambdas from a function, if they
were all unused. Removing *all* value lambdas is no longer
allowed. Instead (\_ -> E) will become (\_void -> E), where it used to
become E. The previous behavior can be recovered via the new
-ffun-to-thunk flag.

Nofib notables:

----------------------------------------------------------------
        Program               O2          O2 newly ignoring RealWorld
                                          and not turning function
                                          closures into thunks
----------------------------------------------------------------

 Allocations

  comp_lab_zift            333090392%           -5.0%
reverse-complem            155188304%           -3.2%

        rewrite             15380888%           +4.0%
         boyer2              3901064%           +7.5%

rewrite previously benefited from fortunate LoopBreaker choice that is
now disrupted.

A function in boyer2 goes from $wonewayunify1 size 700 to size 650,
thus gets inlined into rewritelemmas, thus exposing a parameter
scrutinisation, thus allowing SpecConstr, which unfortunately involves
reboxing.

Run Time

 fannkuch-redux                 7.89%          -15.9%

            hpg                 0.25%           +5.6%
           wang                 0.21%           +5.8%

/shrug