Interactive runner that walks a Technique

[istathar]

Introduces an interactive runner that walks a Technique, prompts the user with the details of the next step, takes their input, then records the result.

This branch adds technique run and technique resume to the command-line interface. The run subcommand reads a Technique document as parameter and starts presenting its steps. the resume subcommand resumes an interrupted run, taking a "run id" parameter to do so. This functionality is carried out in a new top-level runner:: module, with an evaluate() effectively taking an Operation and an Environment and evaluating it to a Value.

The Value type is now defined in a new top-level value:: module, following the naming convention originally established in Technique v0.

This lands an initial local state store for holding PFFTT files, written in .store/ in the present directory. These are named for the Technique document being executed and include basic information recording when each step is completed.

Branch also addresses a few corner cases that emerged while improving test coverage.