Add an in typemap for PROPHASH to the Perl bindings.

* subversion/bindings/swig/include/svn_containers.swg:
  Add the typemap.

* subversion/bindings/swig/perl/libsvn_swig_perl/swigutil_pl.c,
  subversion/bindings/swig/perl/libsvn_swig_perl/swigutil_pl.h:
  Add the corresponding conversion function svn_swig_pl_hash_to_prophash() 
  and the single-item conversion helper convert_pl_svn_string_t().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1479540 13f79535-47bb-0310-9956-ffa450edef68