internal/runtime/maps: initial swiss table map implementation

internal/runtime/maps: initial swiss table map implementation

Add a new package that will contain a new "Swiss Table"
(https://abseil.io/about/design/swisstables) map implementation, which
is intended to eventually replace the existing runtime map
implementation.

This implementation is based on the fabulous
github.com/cockroachdb/swiss package contributed by Peter Mattis.

This CL adds an hash map implementation. It supports all the core
operations, but does not have incremental growth.

For #54766.

Change-Id: I52cf371448c3817d471ddb1f5a78f3513565db41
Reviewed-on: https://go-review.googlesource.com/c/go/+/582415
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>