Made unsafeBuild private[this]
Made unsafeBuild private[this]

Also removed branches for n < 0, since unsafeBuild is always called with a
positive numerator.

guersam/spire,adampingel/spire,woparry/spire,kevinmeredith/spire,woparry/spire,JackSullivan/spire,siddhartha-gadgil/spire,codeaudit/spire,rklaehn/spire,adampingel/spire,AlecZorab/spire,guersam/spire,non/spire,kevinmeredith/spire,tixxit/spire,tixxit/spire,JackSullivan/spire,siddhartha-gadgil/spire,non/spire,rklaehn/spire,AlecZorab/spire,codeaudit/spire