Add examples

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Add the following examples to the docs and test them.

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import ZZ from '@aureooms/js-integer-ring' ;
const QQ =  RationalField(ZZ) ;
export default QQ ;

QQ.has(1) ; // true
const x = ZZ.get('-123').pown(53) ;
QQ.has(x) ; // true
QQ.get([2, -4]).toString() ; // -1/2
QQ.get([x.muln(3), x.muln(-12)]).toString() ; // -1/4

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const x = Symbol('x') ;
KX = QQ[x] ;

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Implement .toFixed([digits]). See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toFixed. Q: Handle radix parameter?

const x = QQ.get(1,2) ;
x.toFixed() ; // 0.5
const y = QQ.get(1,6) ;
y.toFixed() ; // 0.1|6
const z = QQ.get(1,7) ;
z.toFixed() ; // 0.|142857
z.toFixed(3) ; // 0.142
z.toFixed(10) ; // 0.1428571428

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QQ.get(22,7).sub(QQ.get(1,791)).toString() ; // 355/113

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QQ.get('0.|142857').toString(); // 1/7