- rename Context to exprContext to remove it from public interface   (and to match the style of declContext)
- rename Context to exprContext to remove it from public interface
  (and to match the style of declContext)

R=rsc
http://go/go-review/1024033
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go