text/template: Reconsider new IsTrue API

[dsymonds]

In https://go-review.googlesource.com/#/c/14593/, to address #12033, @robpike added a new function, func IsTrue(val reflect.Value) (truth, ok bool) to text/template. That's the first use of reflect in this package's API, thus pulling in ~16 KB of new package export data.

An alternative is to change this new function to be func IsTrue(val interface{}) (truth, ok bool), and do the reflection inside the text/template package.

After a brief discussion with @adg, he asked me to file an issue to track this.

[adg]

The proposed alternative seems more generally useful.​