Issue #2377: Make importlib the implementation of __import__().

importlib._bootstrap is now frozen into Python/importlib.h and stored
as _frozen_importlib in sys.modules. Py_Initialize() loads the frozen
code along with sys and imp and then uses _frozen_importlib._install()
to set builtins.__import__() w/ _frozen_importlib.__import__().
diff --git a/Python/pystate.c b/Python/pystate.c
index 31b5423..fdcbbce 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -79,6 +79,7 @@
         interp->codec_error_registry = NULL;
         interp->codecs_initialized = 0;
         interp->fscodec_initialized = 0;
+        interp->importlib = NULL;
 #ifdef HAVE_DLOPEN
 #ifdef RTLD_NOW
         interp->dlopenflags = RTLD_NOW;
@@ -116,6 +117,7 @@
     Py_CLEAR(interp->modules_reloading);
     Py_CLEAR(interp->sysdict);
     Py_CLEAR(interp->builtins);
+    Py_CLEAR(interp->importlib);
 }