Don't use eval when evaluating RealMath subclasses



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By the time this `compute_one` function is called the `t.sum()` should already have been turned into a number.  Compute_one functions are given the already-computed versions of the children of the expression.  This is handled in [bottom up](https://github.com/ContinuumIO/blaze/blob/master/blaze/compute/core.py#L53).