Add simple fragment to GAE for performance upgrade

ProjectedFragment is trying to mimic the ImmutableFragment, but with inevitable overhead on memory pressure.

We should keep a version of ImmutableFragment in GAE (or import from libgrape-lite) to keep the ability to reproduce the best performance reported in our benchmark of libgrape-lite.

Start by locating ProjectedFragment and ImmutableFragment in the GAE code, then inspect how libgrape-lite exposes the latter and review the referenced benchmark. Done means GAE has an ImmutableFragment version or import path that preserves the ability to reproduce the reported libgrape-lite performance.

Written by the indexing model from the issue text.