Visible forall in types of terms, Part 2

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.

See also https://gitlab.haskell.org/ghc/ghc/-/wikis/DH-current-status.

The first part was tracked at #22326 (closed) and implemented in 33b6850a. This new ticket is to track the implementation of the remaining parts of the proposal, i.e. Part 2. To that end, I identified the following subtasks:

• #23738 (closed) T2T (term-to-type) transformation in expressions (at call sites)
• #23739 (closed) T2T (term-to-type) transformation in patterns (at binding sites)
• #24159 (closed) The types-in-terms syntax (forall, ->, =>)
• #23719 (closed) Make forall a keyword
• #23740 (closed) Term variable capture and implicit quantification

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 a x = x :: a
  
  -- Usage:
  n = idv 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 a = typeRep @a
  
  -- Usage:
  t = typeRepVis (Maybe String)

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

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

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

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