Optimizations for bytes reallocation.

This uses up to 12.5% overallocation, not entirely unlike list_resize().
Could probably use more tweaks for odd allocation patterns, TBD.

Also add __alloc__() method which returns the actually allocated size.

PS. I'm now convinced that we need something like "".join(); later.

Include/bytesobject.h

@@ -21,6 +21,7 @@ extern "C" {
 /* Object layout */
 typedef struct {
     PyObject_VAR_HEAD
+    Py_ssize_t ob_alloc; /* How many bytes allocated */
     char *ob_bytes;
 } PyBytesObject;