archive/zip: add Writer.Flush

This is needed for callers to be able to keep track of the
writing position within a zip file. Otherwise it's not
possible to compute the size of headers, and the TOC isn't
written until the very end.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/134210043
runtime: remove old malloc test programs

These haven't been run in ages.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews, iant, khr, r
https://codereview.appspot.com/134170043