More refactoring to support global variable optimization.
        
        Changed SymbolTable to use RefPtr<UString::Rep> as its key instead of
        UString::Rep*. With globals, the symbol table can outlast the
        declaration node for any given symbol, so the symbol table needs to ref
        its symbol names.
        
        In support, specialized HashMaps with RefPtr keys to allow lookup
        via raw pointer, avoiding refcount churn.
        
        SunSpider reports a .6% speedup (prolly just noise).

        * JavaScriptCore.vcproj/WTF/WTF.vcproj: Added new file: wtf/RefPtrHashMap.h
        * JavaScriptCore.xcodeproj/project.pbxproj: ditto

        * kjs/JSVariableObject.cpp:
        (KJS::JSVariableObject::getPropertyNames): Symbol table keys are RefPtrs now.

        * kjs/SymbolTable.h: Modified key traits to match RefPtr. Added a
        static Rep* for null, which helps compute the deletedValue() trait.

        * wtf/HashMap.h: #include the RefPtr specialization so everyone can use it.

        * wtf/RefPtrHashMap.h: Copied from wtf/HashMap.h. Added overloaded versions
        of find(), contains(), get(), set(), add(), remove(), and take() that take
        raw pointers as keys.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@28777 268f45cc-cd09-0410-ab3c-d52691b4dbfc