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.

Include/stringobject.h

@@ -177,16 +177,26 @@ PyAPI_FUNC(int) PyString_AsStringAndSize(
 				   strings) */
     );
 
+
 /* Using the current locale, insert the thousands grouping
    into the string pointed to by buffer.  For the argument descriptions,
    see Objects/stringlib/localeutil.h */
+PyAPI_FUNC(Py_ssize_t) _PyString_InsertThousandsGroupingLocale(char *buffer,
+                                  Py_ssize_t n_buffer,
+                                  char *digits,
+                                  Py_ssize_t n_digits,
+                                  Py_ssize_t min_width);
 
-PyAPI_FUNC(int) _PyString_InsertThousandsGrouping(char *buffer,
-						  Py_ssize_t n_buffer,
-						  Py_ssize_t n_digits,
-						  Py_ssize_t buf_size,
-						  Py_ssize_t *count,
-						  int append_zero_char);
+/* Using explicit passed-in values, insert the thousands grouping
+   into the string pointed to by buffer.  For the argument descriptions,
+   see Objects/stringlib/localeutil.h */
+PyAPI_FUNC(Py_ssize_t) _PyString_InsertThousandsGrouping(char *buffer,
+                                  Py_ssize_t n_buffer,
+                                  char *digits,
+                                  Py_ssize_t n_digits,
+                                  Py_ssize_t min_width,
+                                  const char *grouping,
+                                  const char *thousands_sep);
 
 /* Format the object based on the format_spec, as defined in PEP 3101
    (Advanced String Formatting). */