Move requirements from modules to requirements

[michael-n-cooper]

This moves the requirements from each of the modules into the separate requirements documents. It also separates use cases and requirements into separate sections. Still to do on a future pass:

• Reword use cases to describe actual user stories.
• Reword requirements to be more high level and not simply tautologically require the feature that is proposed in the module as well as, where needed, more clearly explain how they fulfill the use cases.
• Add requirements for vocabulary structure and host language implementation, and probably related use cases.
• Add introduction to requirements document.

[sonya1st]

I keep having to register