Implement a dict_hash_ext function able to deal with non-null-terminated strings...

Implement a dict_hash_ext function able to deal with non-null-terminated strings so that we can use the version written by Christophe in dict_get, where we assume that the string is null-terminated and don't want to pay a strlen for nothing

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@2731 48e7efb5-ca39-0410-a469-dd3cf9ba447f