Add search for retainers by info table

This is my attempt to fix #13 (closed). I mostly cargo-culted what was there already, and when trying this out it seemed to find some results but not as many as I'd expect, so I'd appreciate review from someone who knows what they are doing.

client/src/GHC/Debug/Retainers.hs

 {-# LANGUAGE ViewPatterns #-}
 -- | Functions for computing retainers
-module GHC.Debug.Retainers(findRetainersOf, findRetainersOfConstructor, findRetainersOfConstructorExact, findRetainers, addLocationToStack, displayRetainerStack, addLocationToStack', displayRetainerStack') where
+module GHC.Debug.Retainers(findRetainersOf, findRetainersOfConstructor, findRetainersOfConstructorExact, findRetainersOfInfoTable, findRetainers, addLocationToStack, displayRetainerStack, addLocationToStack', displayRetainerStack') where
 
 import GHC.Debug.Client
 import Control.Monad.State
@@ -45,6 +45,12 @@ findRetainersOfConstructorExact limit rroots clos_name =
 
           return $ (infoName cur_loc) == clos_name
 
+findRetainersOfInfoTable :: Maybe Int -> [ClosurePtr] -> InfoTablePtr -> DebugM [[ClosurePtr]]
+findRetainersOfInfoTable limit rroots info_ptr =
+  findRetainers limit rroots go
+  where
+    go _ sc = return $ tableId (info (noSize sc)) == info_ptr
+
 -- | From the given roots, find any path to one of the given pointers.
 -- Note: This function can be quite slow! The first argument is a limit to
 -- how many paths to find. You should normally set this to a small number