crypto/internal/mlkem768: make Decapsulate a method

crypto/internal/mlkem768: make Decapsulate a method

This will make it easier to support multiple sizes if needed.

Change-Id: I47495559fdbbf678fd98421ad6cb28172e5c810d
Reviewed-on: https://go-review.googlesource.com/c/go/+/621977
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>