This patch fixes a bad omission in CSE, thanks to 'michaelt' for spotting
it, and correctly identifying the fix (in cseRhs).  The trouble was with
   x1 = C a b
   x2 = C x1 b
   y1 = C a b
   y2 = C y1 b
we were not commoning up y2=x2, because we failed to substitute y1:=x1,
so y2's RHS looked different to x2's

I also refactoring, so taht the cs_map in a CSEnv map is
       cs_map    :: CoreMap (OutExpr, Id)
instead of
       cs_map    :: CoreMap (OutExpr, OutExpr)
Much nicer!

This doesn't make much difference to allocation, but it gives a surprisingly
big benefit to binary size.

--------------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
--------------------------------------------------------------------------------
           ansi          -1.7%     -0.8%      0.00      0.00     +0.0%
           bspt          -1.6%     -1.5%      0.01      0.01     +0.0%
      cacheprof          -1.8%     -0.2%     +1.6%     +1.9%     +2.7%
            fft          -1.4%     -1.3%      0.06      0.06    +11.1%
            ida          -1.4%     -1.0%      0.12      0.12     +0.0%
           rfib          -1.4%     -0.1%      0.03      0.03     +0.0%
            scs          -1.6%     -0.1%     +1.5%     +1.5%     +0.0%
  spectral-norm          -1.3%     -0.1%     -0.2%     -0.2%     +0.0%
            tak          -1.4%     -0.1%      0.02      0.02     +0.0%
        veritas          -1.4%     -0.1%      0.00      0.00     +0.0%
--------------------------------------------------------------------------------
            Min          -2.5%     -1.5%    -11.8%    -11.8%     -8.0%
            Max          -1.0%     +0.0%     +2.7%     +2.5%    +11.1%
 Geometric Mean          -1.3%     -0.1%     -2.6%     -2.6%     +0.0%