Add .fn attributes to cache wrappers in caching.py (#88)

This will enable `qcore.get_original_fn` (and thence pyanalyze) to understand the signatures of these functions.

qcore/caching.py

@@ -271,6 +271,7 @@ def wrapper(*args, **kwargs):
                 return value
 
         wrapper.clear = cache.clear
+        wrapper.fn = fn
 
         return wrapper
 
@@ -313,6 +314,7 @@ def new_fun(self, *args, **kwargs):
 
         # just so unit tests can check that this is cleaned up correctly
         new_fun.__cached_per_instance_cache__ = cache
+        new_fun.fn = fun
         return new_fun
 
     return cache_fun
@@ -379,6 +381,7 @@ def clear_cache():
 
     new_fun.__cache = {}
     new_fun.clear_cache = clear_cache
+    new_fun.fn = fun
     return new_fun
 
 
@@ -442,6 +445,7 @@ def dirty(*args, **kwargs):
         new_fun.__cache_times = {}
         new_fun.clear_cache = clear_cache
         new_fun.dirty = dirty
+        new_fun.fn = fun
         return new_fun
 
     return cache_fun