Add an example of generating code and consuming it as a library.

[aiuto]

This is a toy example that shows

• a rule that builds a binary that can generate some code from an input file
• using that rule to create python and C++ source files
• which are consumed by *_library rules
• and, finally, by *_binary rules.

[gregestren]

Always happy to see new examples!

Was this inspired by a particular use case? I'm asking because I'd like to have some rough taxonomy of the categories we want to document and why.