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.

Lib/test/test_format.py

@@ -232,6 +232,10 @@ class FormatTest(unittest.TestCase):
         testboth("%o", -042L, "-42")
         testboth("%o", float(042), "42")
 
+        # alternate float formatting
+        testformat('%g', 1.1, '1.1')
+        testformat('%#g', 1.1, '1.10000')
+
         # Test exception for unknown format characters
         if verbose:
             print 'Testing exceptions'