Rename "static" directory to "assets".
Replace "asset directory" concept with manifest of assets.

We were partway here already: the client served assets from the manifest instead
of from a static directory (since 5b8e1c1), and we already generated the list of
assets in the slice JSON file. But on the server, we ignored that list and
re-walked the asset directory at bundle time.

Now, the idea of asset directory is solely a part of initFromAppDir.

This also fixes a bug where assets that weren't associated with on-disk files
wouldn't work properly if Asset.get* is called in a package loaded with
unipackage.load. Not really sure how dev-bundle-fetcher even worked...

Also fixes a bug in builder where generated filenames didn't actually avoid
duplicate files.

This does not bump BUILT_BY because the previous commit did, and this commit
will not be pushed without the previous commit.
Rename "build version" to "built by" in other places too.