Add 'clist' string encoding, to build strings char by char.

The \str_convert:Nnnn function now accepts an input or
output encoding equal to 'clist', which expects or produces
a comma-delimited list of character codes.  This allows to
build a string from its character codes, and to perform some
operations on Unicode strings even with an 8-bit engine.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4484 de43f980-851b-0410-b2f7-c40aca1f87e0