sum-types: Rename Newtype to NominalType

[RyanGlScott]

On the sum-types branch (#1602), we have this data type:

cryptol/src/Cryptol/TypeCheck/Type.hs

Lines 320 to 327 in 221bf4a

	-- | Nominal types
	data Newtype = Newtype
	{ ntName :: Name
	, ntParams :: [TParam]
	, ntConstraints :: [Prop]
	, ntDef :: NewtypeDef
	, ntDoc :: Maybe Text
	} deriving (Show, Generic, NFData)

Although it is named "Newtype", this is quite misleading, as this now encompasses both newtypes and enums. To make this more obvious, we should rename Newtype to NominalType. We should also rename related definitions (e.g., NewtypeDef).