crypto/ed25519/internal/edwards25519: sync with filippo.io/edwards25519

Import the following commits (and minor comment fixes):

    * 17a0e59 - field: fix heap escape in SqrtRatio <Filippo Valsorda>
    * edec5b9 - field: fix SqrtRatio when arguments and receiver alias <Filippo Valsorda>
    * 26ce6fc - edwards25519: expand the SetUniformBytes docs <Filippo Valsorda>
    * c1c1311 - edwards25519: make Scalar and field.Element setters return errors <Filippo Valsorda>

Change-Id: I102eb04818a2bed43467f3eda6fd4c46b09878fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/373077


Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>