Move string conversion functions to a separate module.

The l3str.dtx package now only contains functions such
as \str_new:N or \str_count_spaces:n which should be
fairly uncontroversial and stable.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4407 de43f980-851b-0410-b2f7-c40aca1f87e0