runtime: verify hmap type size
runtime: verify hmap type size

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, khr
https://codereview.appspot.com/114510044

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go