Instead of having functions for 0 to 5 arguments, we now have a
variadic template, allowing for an arbitrary number of arguments.
I also refactored the internal FmtElm class to only include the type
field if building for debug, because it is only used in this setting.

R=titzer@chromium.org, ahaas@chromium.org

Review-Url: https://codereview.chromium.org/2507093004
Cr-Commit-Position: refs/heads/master@{#41099}