Use a newtype `Code` for the return type of typed quotations (Proposal #195)

There are three problems with the current API:

1. It is hard to properly write instances for ``Quote m => m (TExp a)`` as the type is the composition
   of two type constructors. Doing so in your program involves making your own newtype and
   doing a lot of wrapping/unwrapping.

   For example, if I want to create a language which I can either run immediately or
   generate code from I could write the following with the new API. ::

      class Lang r where
        _int :: Int -> r Int
        _if  :: r Bool -> r a -> r a -> r a

      instance Lang Identity where
        _int = Identity
        _if (Identity b) (Identity t) (Identity f) = Identity (if b then t else f)

      instance Quote m => Lang (Code m) where
        _int = liftTyped
        _if cb ct cf = [|| if $$cb then $$ct else $$cf ||]

2. When doing code generation it is common to want to store code fragments in
   a map. When doing typed code generation, these code fragments contain a
   type index so it is desirable to store them in one of the parameterised
   map data types such as ``DMap`` from ``dependent-map`` or ``MapF`` from
   ``parameterized-utils``.

   ::

      compiler :: Env -> AST a -> Code Q a

      data AST a where ...
      data Ident a = ...

      type Env = MapF Ident (Code Q)

      newtype Code m a = Code (m (TExp a))

   In this example, the ``MapF`` maps an ``Ident String`` directly to a ``Code Q String``.
   Using one of these map types currently requires creating your own newtype and constantly
   wrapping every quotation and unwrapping it when using a splice. Achievable, but
   it creates even more syntactic noise than normal metaprogramming.

3. ``m (TExp a)`` is ugly to read and write, understanding ``Code m a`` is
   easier. This is a weak reason but one everyone
   can surely agree with.

Updates text submodule.

1 files changed, 3 insertions, 3 deletions

diff --git a/testsuite/tests/parser/should_compile/Proposal229f_instances.hs b/testsuite/tests/parser/should_compile/Proposal229f_instances.hs
index 2bd5a8ee19..357a53fcb4 100644
--- a/testsuite/tests/parser/should_compile/Proposal229f_instances.hs
+++ b/testsuite/tests/parser/should_compile/Proposal229f_instances.hs
@@ -8,8 +8,8 @@ import Data.String
 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax
 
-instance IsList (Q (TExp String)) where
-  type Item (Q (TExp String)) = Char
+instance IsList (Code Q String) where
+  type Item (Code Q String) = Char
   fromList = liftTyped
   toList = undefined
 
@@ -18,7 +18,7 @@ instance IsList (Q Exp) where
   fromList = lift
   toList = undefined
 
-instance IsString (Q (TExp String)) where
+instance IsString (Code Q String) where
   fromString = liftTyped
 
 instance IsString (Q Exp) where