Add polynomial context switcher

[tlepoint]

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[tlepoint]

The polynomial extender is slower than I would like too. In my original implementation, to go from 3 to 6 moduli, it was taking

poly::extend/1024/3->6  time:   [62.700 us 62.733 us 62.773 us]

and this implementation is

rq_switchers/extender/1024/3_to_6                                                                            
                        time:   [96.108 µs 96.225 µs 96.368 µs]

It is noticeably higher, so we need to investigate more. Part of it is due to the use of u256 instead of ~u192, but I am not sure it accounts for all the difference.