reflect: stop using run-time code generation

Step 3 of http://golang.org/s/go11func.

Fixes issue 3736.
Fixes issue 3738.
Fixes issue 4081.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7393050
runtime: delete old closure code

Step 4 of http://golang.org/s/go11func.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7393049