Integer#toString has side effects depending on the base

[make-github-pseudonymous-again]

const x = ZZ.from(10000) ;
x.toString(10); // 10000
x.toString(10); // 10000
x.toString(16); // 2710
x.toString(16); // 2

[make-github-pseudonymous-again]

The culprit is https://github.com/aureooms/js-integer-big-endian/blob/57303750241797740359d0e41d86c526688b1d76/src/core/convert/_convert_slow.js#L30 . Need to find an elegant/efficient way to allow copying the input array in this case. Should avoid unnecessary copies.