Rename `mapitervalue` → `mapiterelem` in the `reflect` package.
Rename `mapitervalue` → `mapiterelem` in the `reflect` package.

This reflects a change in the upstream intended to make use of terms
"value" and "element" more consistent in the context of maps.

gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs