Signature maps should only be updated, but never copied. We had a bug
because we accidentally updated a copy of the map. This refactoring
prevents any such bugs in the future, and fixes more occurences where
we accidentally copied structs containing a signature map (the move-only
constraint also extends to all structs containing a signature map).

Drive-by: Make InstanceBuilder::NeedsWrappers const.

R=titzer@chromium.org

Bug: chromium:741750
Change-Id: Id919203d8c4078e608a1163e5c790c97d06a9753
Reviewed-on: https://chromium-review.googlesource.com/571791Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46728}