Remove the manual 2.718... from the code
Use BigDecimal#div and #mult to control the precision

* using BigDecimal / BigDecimal yields a 200 precision result
  and then z * z even a 400 digits precision result; this
  made the calculation absurdly slow (many seconds for one
  calculation).
* using BigDecimal#div and #mult allows explicit setting of
  the precision (like in the C code, div2 and mult2)
* now these tests (that are currently activated) pass in 0.24 seconds
  bin/jruby -S rake test:mri TEST=test/mri/bigdecimal/test_bigdecimal.rb