Add test for big curve multiplication. Benchmarks

FFInt:
* divide: 4 seconds
* multiply big: 3.5
* inverse: 1.5
* multiply with large remainder: 3.7
* subtract: 9

Curve:
* multiply tiny numbers: 30 seconds
* add: 11 seconds
* double: 13 seconds
* negate: 2.5 seconds

testMultiplyBig still takes forever and throws an overflow after a few
minutes.