The other half of Issue #1580: use short float repr where possible.

Addresses the float -> string conversion, using David Gay's code which
was added in Mark Dickinson's checkin r71663.

Also addresses these, which are intertwined with the short repr
changes:

- Issue #5772: format(1e100, '<') produces '1e+100', not '1.0e+100'
- Issue #5515: 'n' formatting with commas no longer works poorly
    with leading zeros.
- PEP 378 Format Specifier for Thousands Separator: implemented
    for floats.

Objects/bytesobject.c

@@ -562,6 +562,7 @@ PyBytes_AsStringAndSize(register PyObject *obj,
 /* -------------------------------------------------------------------- */
 /* Methods */
 
+#include "stringlib/stringdefs.h"
 #define STRINGLIB_CHAR char
 
 #define STRINGLIB_CMP memcmp