Recipe for coupled pallets

[JoshOrndorff]

This PR adds a few recipes necessary to demonstrate the notion of "pallet coupling", the practice of allowing pallets to work together.

Addresses #145, #56, and the needs of @siman and Chris D

• [x] Rework vec-set pallet to store a sorted vec of Account Ids
  • [x] tests
  • [x] writeup
• [x] Add map-set pallet to store set of Account Ids as map keys
  • [x] tests
  • [x] writeup
• [x] Introduce AccountSet trait for any type that can supply a set of accounts
• [x] Implement AccountSet trait for both pallets
• rework check-membership into two pallets
  • [x] One that is tightly coupled to the vec-set
  • [x] tests
  • [x] One that is loosely coupled throug hthe trait
  • [x] tests
  • [x] writeup
• [x] Install pallets in super-runtime

[subsocialdev]

Very useful recipe!

[subsocialdev]

There is a check for MAX_MEMBERS in vec-set but not in map-set

[dvdplm]

The Ci failure seems worth looking into, missing file something something?