Hash::FieldHash provides the field hash mechanism which supports the
 inside-out technique.
 .
 You may know Hash::Util::FieldHash. It's a very useful module, but too
 complex to understand all the functions.
 .
 This is an alternative to Hash::Util::FieldHash with following features:
 .
 Simpler interface
 .
   Hash::FieldHash provides a few functions: fieldhash() and fieldhashes().
   That's enough.
 .
 Higher performance
   Hash::FieldHash is faster than Hash::Util::FieldHash, because its internals
   use simpler structures.