Add Gob encoding functions for Key.
Add Gob encoding functions for Key.

This is so that *Key can transparently be encoded in tumble mutation structs.

R=dnj@chromium.org, estaab@chromium.org, maruel@chromium.org, tandrii@chromium.org, vadimsh@chromium.org
BUG=533020

Review URL: https://codereview.chromium.org/1397673003

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go