exp/template: fixes and updates.

exp/template: fixes and updates.
- fix line numbers - forgot to update state.line during execution
- add a comment convention {{/* comment */}}
- set.Template returns the named template in the set
- set.Execute executes the named template in the set
- use a local methodByName so this package can be used with earlier release of reflect.
- use initial cap to detect exported names

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4668054