Finish the apps/rdd-ops-safe example

[facundominguez]

Support for linear types when working with streams needs to be implemented or worked-around to finish the conversion of the example from the unsafe to safe (linear) interface.

This issue is a follow up of #160

[aspiwack]

Is the linear stream sub-library from linear-base sufficient to address this?

[facundominguez]

It may. We need to get into writing a safe interface for jvm-streaming to understand the difficulties.