String alloc shouldn't convert to std::string.

As I work through making code use more C++ RAII and such, most
of the work is handling strings, especially temporaries.  As member
variables which manage string memory get turned into `std::string`,
some use cases might wind up leaking memory.  (One was found in
this change.)

By using a non-convertible-to-string result, such accidents should
be avoided.

dw/hyphenator.cc

@@ -371,7 +371,7 @@ TrieBuilder::~TrieBuilder ()
 void TrieBuilder::insert (const char *key, const char *value)
 {
    dataList->increase ();
-   dataList->getLastRef ()->key = (unsigned char *) dStrdup(key);
+   dataList->getLastRef ()->key = (unsigned char *) dStrdup(key).ptr;
    dataList->getLastRef ()->value = dataZone->strdup (value);
 }