Visible forall in types of terms, Part 1

GHC Proposal #281: Visible forall in types of terms is presented in two parts:

1. Part 1 introduces the forall a -> quantifier in types of terms but also requires a syntactic marker at use sites.
2. Part 2 specifies when it is permissible to omit the type herald.

This issue is to track the implementation of Part 1 only. Here are a few examples of code that we aim to allow:

• A variant of id that uses visible forall:

  -- Definition:
  idv :: forall a -> a -> a
  idv (type a) x = x :: a
  
  -- Usage:
  n = idv (type Double) 42

  This is equivalent to n = (42 :: Double).

• A wrapper around typeRep that uses visible forall:

  -- Definition:
  typeRepVis :: forall a -> Typeable a => TypeRep a
  typeRepVis (type a) = typeRep @a
  
  -- Usage:
  t = typeRepVis (type (Maybe String))

• A wrapper around sizeOf that uses visible forall instead of Proxy:

  -- Definition:
  sizeOfVis :: forall a -> Storable a => Int
  sizeOfVis (type a) = sizeOf (undefined :: a)
  
  -- Usage:
  n = sizeOfVis (type Int)

• A wrapper around symbolVal that uses visible forall instead of Proxy:

  -- Definition:
  symbolValVis :: forall s -> KnownSymbol s => String
  symbolValVis (type s) = symbolVal (Proxy :: Proxy s)
  
  -- Usage
  str = symbolValVis (type "Hello, World")