ENH: Add `polyrootval` to numpy.polynomial

As one can easily encounter when working with high-order signal processing
filters, converting a high-order polynomial from its roots to its polynomial
coefficients can be quite lossy, leading to inaccuracies in the filter's
properties.

This PR adds a new function, `polyrootval` - based on `polyval` -  that
evaluates a polynomial given a list of its roots. The benefit of calculating it
this way can be seen at scipy/scipy:6059. Some tests are included, as well.

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