Summary:
This commit implements the plan in #13140:

* Today, roles in signature files default to representational. Let's change the
  default to nominal, as this is the most flexible implementation side. If a
  client of the signature needs to coerce with a type, the signature can be
  adjusted to have more stringent requirements.

* If a parameter is declared as nominal in a signature, it can be implemented
  by a data type which is actually representational.

* When merging abstract data declarations, we take the smallest role for every
  parameter. The roles are considered fix once we specify the structure of an
  ADT.

* Critically, abstract types are NOT injective, so we aren't allowed to
  make inferences like "if T a ~R T b, then a ~N b" based on the nominal
  role of a parameter in an abstract type (this would be unsound if the
  parameter ended up being phantom.)  This restriction is similar to the
  restriction we have on newtypes.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, bgamari, austin, goldfire

Subscribers: goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D3123