Replace use of rb_float_new/rb_num2dbl with DBL2NUM and NUM2DBL.
Replace use of rb_float_new/rb_num2dbl with DBL2NUM and NUM2DBL.

This is mostly to aid in porting back to earlier versions of Ruby or
other implementations, if I ever get around to doing that, since I'm not
sure I can guarantee rb_float_new or rb_num2dbl will exist, whereas it
seems like the TYP2TYP macros are like band-aids over possibly-changing
APIs.

nilium/ruby-snowmath,nilium/ruby-snowmath