Two speedup hacks. Caching the hash saves recalculation of a string's

Two speedup hacks.  Caching the hash saves recalculation of a string's
hash value.  Interning strings (which requires hash caching) tries to
ensure that only one string object with a given value exists, so
equality tests are one pointer comparison.  Together, these can speed
the interpreter up by as much as 20%.  Each costs the size of a long
or pointer per string object.  In addition, interned strings live
until the end of times.  If you are concerned about memory footprint,
simply comment the #define out here (and rebuild everything!).