Implement fetch engine core method

[tgvashworth]

• shouldFetch
  • if false, bail with CancelledError
• getRequest
  • produces a Promise for a Request
• willFetch
  • side-effects, ignore return value
• (internal-only) fetch
  • make the Request with an inner fetch implementation
• fetch
  • allow plugins to cancel
• getResponse
  • produce a Promise for a Response
• didFetch
  • side-effects only, ignore return value