Move partition and rpartition to new strmisc module

This was done because partition is an uncommonly used proc that is still
useful in rare cases. There is also a desire to add more procs to this
module at a later time.

web/news/version_0_15_released.rst

@@ -17,6 +17,15 @@ Library Additions
 - Added ``readHeaderRow`` and ``rowEntry`` to ``parsecsv.nim`` to provide
   a lightweight alternative to python's ``csv.DictReader``.
 
+- Added ``center``, ``rsplit``, and ``expandTabs`` to ``strutils.nim`` to
+  provide similar Python functionality for Nim's strings.
+
+- Added ``isTitle``, ``title``, and ``swapCase`` to ``unicode.nim`` to
+  provide unicode aware string case manipulation.
+
+- Added a new module ``lib/pure/strmisc.nim`` to hold uncommon string
+  operations. Currently contains ``partition`` and ``rpartition``.
+
 Compiler Additions
 ------------------