crypto/internal/fips/sha3: restructure as an internal package

crypto/internal/fips/sha3: restructure as an internal package

Main changes are

    - return concrete *Digest and *SHAKE instead of interfaces

    - make tests external (sha3_test) so they will be easy to move to
      the public package

    - drop most of the developer guidance docs (to be updated and
      reintroduced in the public package)

    - consolidate the _noasm.go files (matching the single _s390x.go)

    - move TestAllocations from build tags to testenv

    - temporarily disable s390x code, to refactor in a following CL

For #69536

Change-Id: Ie5fd3e2b589b9eb835b9e3174b7a79c2ac728ab1
Reviewed-on: https://go-review.googlesource.com/c/go/+/617357
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>