Performance tweaks for formula evaluation

We've been using Dentaku to perform some dynamic calculations on data points on an internal project. We pulled in the Ruby math functions but it ran slowly depending the complexity of the formula. I ran it through rubyprof to look at what was taking the most time and applied some tweaks:

Before:
```
Measuring Calculator Performance
                 user     system      total        real
method.call 10.910000   0.190000  11.100000 ( 11.097406)
method.send 17.830000   0.240000  18.070000 ( 18.077954)
```
After:
```
Measuring Calculator Performance
                 user     system      total        real
method.call  3.230000   0.000000   3.230000 (  3.228235)
method.send  5.250000   0.010000   5.260000 (  5.258325)
```

You can run the performance test using this code here: https://gist.github.com/jmangs/cee6dc95811200623551#file-perf-rb