go-ify foldr2

This helps with the changes in #7994, but might also generally be a good
idea (ignore the runtime):

--------------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
           fft2          -0.1%     -1.5%      0.07      0.07     +0.0%
       fibheaps          +0.0%    -17.2%      0.03      0.03     +0.0%
          fluid          +0.5%     -0.7%      0.01      0.01     +0.0%
      integrate          +0.0%     -0.9%      0.16      0.16     +0.0%
        rewrite          +0.0%     -1.1%      0.02      0.02     +0.0%
--------------------------------------------------------------------------------
            Min          -0.1%    -17.2%     -1.6%     +0.0%     +0.0%
            Max          +0.5%     +0.0%   +107.7%   +106.2%    +11.3%
 Geometric Mean          +0.0%     -0.2%    +23.7%    +23.9%     +0.1%