Backport of some of the work in r71665 to trunk. This reworks much of

int, long, and float __format__(), and it keeps their implementation
in sync with py3k.

Also added PyOS_double_to_string. This is the "fallback" version
that's also available in trunk, and should be kept in sync with that
code. I'll add an issue to document PyOS_double_to_string in the C
API.

There are many internal cleanups. Externally visible changes include:

- Implement PEP 378, Format Specifier for Thousands Separator, for
  floats, ints, and longs.

- Issue #5515: 'n' formatting for ints, longs, and floats handles
  leading zero formatting poorly.

- Issue #5772: For float.__format__, don't add a trailing ".0" if
  we're using no type code and we have an exponent.

Objects/stringlib/stringdefs.h

@@ -6,6 +6,15 @@
    compiled as unicode. */
 #define STRINGLIB_IS_UNICODE     0
 
+/* _tolower and _toupper are defined by SUSv2, but they're not ISO C */
+/* This needs to be cleaned up. See issue 5793. */
+#ifndef _tolower
+#define _tolower tolower
+#endif
+#ifndef _toupper
+#define _toupper toupper
+#endif
+
 #define STRINGLIB_OBJECT         PyStringObject
 #define STRINGLIB_CHAR           char
 #define STRINGLIB_TYPE_NAME      "string"
@@ -13,8 +22,8 @@
 #define STRINGLIB_EMPTY          nullstring
 #define STRINGLIB_ISDECIMAL(x)   ((x >= '0') && (x <= '9'))
 #define STRINGLIB_TODECIMAL(x)   (STRINGLIB_ISDECIMAL(x) ? (x - '0') : -1)
-#define STRINGLIB_TOUPPER        toupper
-#define STRINGLIB_TOLOWER        tolower
+#define STRINGLIB_TOUPPER(x)     _toupper(Py_CHARMASK(x))
+#define STRINGLIB_TOLOWER(x)     _tolower(Py_CHARMASK(x))
 #define STRINGLIB_FILL           memset
 #define STRINGLIB_STR            PyString_AS_STRING
 #define STRINGLIB_LEN            PyString_GET_SIZE
@@ -24,5 +33,6 @@
 #define STRINGLIB_CMP            memcmp
 #define STRINGLIB_TOSTR          PyObject_Str
 #define STRINGLIB_GROUPING       _PyString_InsertThousandsGrouping
+#define STRINGLIB_GROUPING_LOCALE _PyString_InsertThousandsGroupingLocale
 
 #endif /* !STRINGLIB_STRINGDEFS_H */