The definition of 'truth' used by if etc. is not trivial to compute, so publish
the implementation to allow custom template functions to have the
same definition as the template language itself.

Fixes #12033.

Change-Id: Icdfd6039722d7d3f984ba0905105eb3253e14831
Reviewed-on: https://go-review.googlesource.com/14593Reviewed-by: Andrew Gerrand <adg@golang.org>