Implement typechecker plugins

Summary:
See https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker

This is based on work by Iavor Diatchki and Eric Seidel.

Test Plan: validate

Reviewers: simonpj, austin

Reviewed By: austin

Subscribers: gridaphobe, yav, thomie, carter

Differential Revision: https://phabricator.haskell.org/D489

Conflicts:
	docs/users_guide/7.10.1-notes.xml
Split SynTyCon to SynonymTyCon and FamilyTyCon

This patch refactors internal representation of type synonyms and type families by splitting them into two separate data constructors of TyCon data type. The main motivation is is that some fields make sense only for type synonyms and some make sense only for type families. This will be even more true with the upcoming injective type families.

There is also some refactoring of names to keep the naming constistent. And thus tc_kind field has become tyConKind and tc_roles has become tcRoles. Both changes are not visible from the outside of TyCon module.

Updates haddock submodule

Reviewers: simonpj

Differential Revision: https://phabricator.haskell.org/D508

GHC Trac Issues: #9812
Kill trailing whitespace
Add toIntegralSized for size-checked conversions between integral types