Refactor Payload.

* Break Payload into a Payload/PayloadField abstraction, and fix the major
  design flaw preventing clean Target subclassing.
* Allow explicit freezing of Payloads and always freeze Payloads in the
  Target superclass `__init__`.
* Tighten up invalidation hashing (fingerprinting) and testing.
* Change the name of "invalidation hash" in general to "fingerprint", since
  the former is ambiguous and used to mean different things in several
  different places.
* Make Repository and Wiki plain objects rather than Target subclasses.
  Users who want to expose a custom Repository instance in all BUILD files now
  register this instance in their internal backend's register.py as an exposed
  object.  Then consumers of this Repository refer to this object directly.
  This allows the removal of the horrible hack in JvmTarget.provides.
  It gets us much closer to having truly immutable Payloads.

Testing Done:
ci.sh is green locally

CI is green: https://travis-ci.org/pantsbuild/pants/builds/36363381

Reviewed at https://rbcommons.com/s/twitter/r/1063/