Rename PyUnicode_AsString -> _PyUnicode_AsString and
PyUnicode_AsStringAndSize -> _PyUnicode_AsStringAndSize to mark
them for interpreter internal use only.

We'll have to rework these APIs or create new ones for the
purpose of accessing the UTF-8 representation of Unicode objects
for 3.1.
diff --git a/Python/symtable.c b/Python/symtable.c
index 0c11ee1..92432ab 100644
--- a/Python/symtable.c
+++ b/Python/symtable.c
@@ -1142,7 +1142,7 @@
 		asdl_seq *seq = s->v.Global.names;
 		for (i = 0; i < asdl_seq_LEN(seq); i++) {
 			identifier name = (identifier)asdl_seq_GET(seq, i);
-			char *c_name = PyUnicode_AsString(name);
+			char *c_name = _PyUnicode_AsString(name);
 			long cur = symtable_lookup(st, name);
 			if (cur < 0)
 				return 0;
@@ -1169,7 +1169,7 @@
 		asdl_seq *seq = s->v.Nonlocal.names;
 		for (i = 0; i < asdl_seq_LEN(seq); i++) {
 			identifier name = (identifier)asdl_seq_GET(seq, i);
-			char *c_name = PyUnicode_AsString(name);
+			char *c_name = _PyUnicode_AsString(name);
 			long cur = symtable_lookup(st, name);
 			if (cur < 0)
 				return 0;