Export generic classes

`beam` relies very heavily on GHC generics, but the standard GHC generic
instances are not appropriate for all use cases (motivating example in
this case being
[`large-records`](https://hackage.haskell.org/package/large-records)).
Fortunately, the way GHC generics is set up allows us to use
non-standard representations, _provided_ that we can then define
additional instances for whatever generic classes are necessary. The
[`beam-large-records`](https://github.com/well-typed/beam-large-records)
library provides integration for `large-records` with `beam-core`, by
providing these additional instances, but this does mean that the
generic classes must be _exported_ from `beam-core` so that additional
instances can be provided. This commit adds these exports.

beam-core/Database/Beam/Backend/SQL/Row.hs

@@ -13,6 +13,9 @@ module Database.Beam.Backend.SQL.Row
   , ColumnParseError(..), BeamRowReadError(..)
 
   , FromBackendRow(..)
+
+    -- * Exported so we can override defaults
+  , GFromBackendRow(..) -- for 'runSelectReturningList' and co
   ) where
 
 import           Database.Beam.Backend.SQL.Types

beam-core/Database/Beam/Schema/Lenses.hs

@@ -4,7 +4,12 @@ module Database.Beam.Schema.Lenses
     ( tableLenses
     , TableLens(..)
 
-    , dbLenses ) where
+    , dbLenses
+
+    -- * Exported so we can override defaults
+    , GTableLenses(..)
+    , GDatabaseLenses(..)
+    ) where
 
 import Database.Beam.Schema.Tables
 

beam-core/Database/Beam/Schema/Tables.hs

@@ -57,7 +57,20 @@ module Database.Beam.Schema.Tables
     , pk
     , allBeamValues, changeBeamRep
     , alongsideTable
-    , defaultFieldName )
+    , defaultFieldName
+
+    -- * Exported so we can override defaults
+    -- For 'Beamable'
+    , GZipTables(..)
+    , GTableSkeleton(..)
+    -- For 'Database'
+    , GZipDatabase(..)
+    -- for 'defaultDbSettings'
+    , GAutoDbSettings(..)
+    , GDefaultTableFieldSettings(..)
+    , ChooseSubTableStrategy
+    , SubTableStrategyImpl
+    )
     where
 
 import           Database.Beam.Backend.Types