This makes the name of the function to construct the map bucket type
consistent with runtimes naming and the existing hmap function.

Change-Id: If4d8b4a54c92ab914d4adcb96022b48d8b5db631
Reviewed-on: https://go-review.googlesource.com/59915
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>