Changed Hasher(32|64) template specialisations from std::string to
std::basic_string<T>. Added additional template specialisations for char
arrays and UnicodeString instances.
Added support for state-based xxhash hashing in schwa hash framework.
Changed return type of xxhash API call to be unsigned.
Second attempt at feature hashtable. Removed key type. Added base class
for entry type to extend upon. Added feature types and instance labels.
Added initial support for representing sparse and dense features
simultaniously.