crypto/internal/edwards25519: move to crypto/internal/fips/edwards25519

crypto/internal/edwards25519: move to crypto/internal/fips/edwards25519

Left most of the tests in for now as they are almost all internal and
hard to externalize.

String initialization in the FIPS module has some issues, so switched
field.TestSqrtRatio to storing decoded byte slices instead.

For #69536

Change-Id: If9e4a2bb780a37a8d102a22ffd13c5293d11a8a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/628776
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Bypass: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>