Also, provide a variadic template Return function for easier use, and
refactor the underlying Return function to not use the Buffer, since
that might still be needed later (for example if trap code is generated
during CallIndirect, and the arguments to the call are stored in the
buffer).

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2591903003
Cr-Commit-Position: refs/heads/master@{#41888}