Rewrite ^ with NaNMath.pow in nanmath-mode

[devmotion]

On the master branch:

julia> using Symbolics

julia> @variables x y;

julia> Base.remove_linenums!(build_function(x^y, [x, y]))
:(function (ˍ₋arg1,)
      begin
          (^)(ˍ₋arg1[1], ˍ₋arg1[2])
      end
  end)

With this PR:

julia> using Symbolics

julia> @variables x y;

julia> Base.remove_linenums!(build_function(x^y, [x, y]))
:(function (ˍ₋arg1,)
      begin
          (NaNMath.pow)(ˍ₋arg1[1], ˍ₋arg1[2])
      end
  end)

[devmotion]

The Symbolics integration test failure seems harmless - apparently there's a test that compares an expression with a reference which includes ^, but with this PR the resulting expression uses (as desired) NaNMath.pow instead of ^.