Issue #13959: Rename imp to _imp and add Lib/imp.py and begin
rewriting functionality in pure Python.

To start, imp.new_module() has been rewritten in pure Python, put into
importlib (privately) and then publicly exposed in imp.
diff --git a/PC/config.c b/PC/config.c
index be51b55..d055f9a 100644
--- a/PC/config.c
+++ b/PC/config.c
@@ -143,7 +143,7 @@
     {"marshal", PyMarshal_Init},
 
     /* This lives it with import.c */
-    {"imp", PyInit_imp},
+    {"_imp", PyInit_imp},
 
     /* These entries are here for sys.builtin_module_names */
     {"__main__", NULL},