Move the codec decode type checks to bytes/bytearray.decode().

Use faster PyUnicode_FromEncodedObject() for bytes/bytearray.decode().

Add new PyCodec_KnownEncoding() API.

Add new PyUnicode_AsDecodedUnicode() and PyUnicode_AsEncodedUnicode() APIs.

Add missing PyUnicode_AsDecodedObject() to unicodeobject.h

Fix punicode codec to also work on memoryviews.
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 7fe4cce..24517e4 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -261,14 +261,10 @@
 
 	codeset = nl_langinfo(CODESET);
 	if (codeset && *codeset) {
-		PyObject *enc = PyCodec_Encoder(codeset);
-		if (enc) {
-			codeset = strdup(codeset);
-			Py_DECREF(enc);
-		} else {
-			codeset = NULL;
-			PyErr_Clear();
-		}
+	    if (PyCodec_KnownEncoding(codeset))
+		codeset = strdup(codeset);
+	    else
+		codeset = NULL;
 	} else
 		codeset = NULL;