Refactor handling of abstract/primitve types

[yav]

PR #1602 adds support for sum types, which reuses a lot of the machinery for newtypes. We should reuse the same machinery for representing abstract types, as they are just another form of a "nominal" type.

[RyanGlScott]

Specifically, we should merge AbstractType:

cryptol/src/Cryptol/TypeCheck/Type.hs

Lines 355 to 364 in 221bf4a

	-- | Information about an abstract type.
	-- XXX: This should probably be combined with newtype as just another
	-- kind of "definition".
	data AbstractType = AbstractType
	{ atName :: Name
	, atKind :: Kind
	, atCtrs :: ([TParam], [Prop])
	, atFixitiy :: Maybe Fixity
	, atDoc :: Maybe Text
	} deriving (Show, Generic, NFData)

Into NewtypeDef:

cryptol/src/Cryptol/TypeCheck/Type.hs

Lines 329 to 331 in 221bf4a

	-- | Definition of a nominal type
	data NewtypeDef = Struct StructCon | Enum [EnumCon]
	deriving (Show, Generic, NFData)

But note that Newtype may be renamed to NominalDef (#1607).