port Code machine

[warner]

https://github.com/warner/magic-wormhole/blob/0.10.5/src/wormhole/_code.py

The Code machine is responsible for acquiring the wormhole code, in one of three modes corresponding to the API calls: w.allocate_code(), w.set_code(), and w.input_code(). The set_code method is trivial, and the other two delegate to the Allocator and Input respectively.

The input_code process is split into phases: first the nameplate is set, then the words are set, so there are separate input events for each phase.

When the code is complete, it notifies the Key machine (which uses it as input to the PAKE function) and the Boss (which notifies the application, which is most important when using allocate_code so it knows what to display to the user for transcription.

[warner]

This port looks good now.