Implement CR#gcd, and fix EuclideanRing[CR]'s methods.
Implement CR#gcd, and fix EuclideanRing[CR]'s methods.

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