text/template: clean up function values

text/template: clean up function values
The recent addition of automatic function invocation generated
some troublesome ambiguities. Restore the previous behavior
and compensate by providing a "call" builtin to make it easy to
do what the automatic invocation did, but in a clear and explicit
manner.

Fixes #3140.

At least for now.

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/5720065