gcdCoefficents improvements

[d-torrance]

A few related changes:

• GMP has a built-in extended GCD function, so we use it instead of the current one that's implemented at top-level, improving performance:

  Before

  i1 : elapsedTime gcdCoefficients(46, 240)
   -- .000109342s elapsed
  
  o1 = {2, 47, -9}
  
  o1 : List

  After

  i1 : elapsedTime gcdCoefficients(46, 240)
   -- .00001076s elapsed
  
  o1 = {2, 47, -9}
  
  o1 : List

• gcdCoefficients(ZZ, RingElement) and gcdCoefficients(RingElement, ZZ) have now been defined, promoting the integer to appropriate ring just like gcd.

• The documentation has been updated (converting to SimpleDoc, adding the new methods, and adding some examples).