Implement Numeric#step.size

Gets Numeric#step.size doing its thing by putting the following pieces
in place:
* implement Numeric#intervalStepSize closely following the MRI
implementation (https://github.com/ruby/ruby/blob/09b02349c212cac6395e9a634e3d4610e9bbc48c/numeric.c#L1799)
* implement Numeric#floatStepSize which Numeric#intervalStepSize relies
on, again porting from MRI (https://github.com/ruby/ruby/blob/09b02349c212cac6395e9a634e3d4610e9bbc48c/numeric.c#L1748))
* refactor Numeric#floatStep19 to also use the new floatStepSize, which
not only avoids duplication, but also fixes some tests
* create Numeric#stepSize, and use it to enumeratorizeWithSize our steps

We'll be able to un-exclude test_size_for_step from test_enumerator.rb
once Range#size is implemented (https://github.com/jruby/jruby/issues/1252).