patch for sharing single character Unicode objects.

Martin's patch had to be reworked in a number of ways to take Unicode
resizing into consideration as well. Here's what the updated patch
implements:

* Single character Unicode strings in the Latin-1 range are shared
  (not only ASCII chars as in Martin's original patch).

* The ASCII and Latin-1 codecs make use of this optimization,
  providing a noticable speedup for single character strings. Most
  Unicode methods can use the optimization as well (by virtue
  of using PyUnicode_FromUnicode()).

* Some code cleanup was done (replacing memcpy with Py_UNICODE_COPY)

* The PyUnicode_Resize() can now also handle the case of resizing
  unicode_empty which previously resulted in an error.

* Modified the internal API _PyUnicode_Resize() and
  the public PyUnicode_Resize() API to handle references to
  shared objects correctly. The _PyUnicode_Resize() signature
  changed due to this.

* Callers of PyUnicode_FromUnicode() may now only modify the Unicode
  object contents of the returned object in case they called the API
  with NULL as content template.

Note that even though this patch passes the regression tests, there
may still be subtle bugs in the sharing code.