Implement pointer mixture network

Here we can see the mixture network is assigning probability mass
to a new, project-specific name by reference https://i.imgur.com/6Zbs2qf.png.

I also took this opportunity to decrease model size targeting 100M, in line
with our original size goals.

My initial strategy was to implement a separate pointer network
in https://dart-review.googlesource.com/c/sdk/+/117005 but having
a single network that can assign probability mass across local
references and vocabulary lexemes is better since

1) only one network and model file
2) no need to coalesce predictions from multiple models

Change-Id: I23cfc2ece61ce30bb69785149a5a6cf1604af18d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121461
Commit-Queue: Ari Aye <ariaye@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>