Replace UString::Rep implementation, following introduction of ropes to JSC.

Reviewed by Darin Adler.

JavaScriptCore: 

    * Remove redundant overcapacity mechanisms.
    * Reduce memory cost of Rep's.
    * Add an inline storage mechanism akin to that in WebCore's StringImpl.

~1% Sunspider progression.

* JavaScriptCore.exp:
* JavaScriptCore.xcodeproj/project.pbxproj:
* runtime/JSString.cpp:
(JSC::JSString::resolveRope):
* runtime/SmallStrings.cpp:
(JSC::SmallStringsStorage::SmallStringsStorage):
* runtime/UString.cpp:
(JSC::initializeUString):
(JSC::createRep):
(JSC::UString::createFromUTF8):
(JSC::UString::createUninitialized):
(JSC::UString::spliceSubstringsWithSeparators):
(JSC::UString::replaceRange):
(JSC::UString::ascii):
(JSC::UString::operator=):
(JSC::UString::toStrictUInt32):
(JSC::equal):
* runtime/UString.h:
(JSC::UString::isEmpty):
(JSC::UString::cost):
(JSC::makeString):
* runtime/UStringImpl.cpp: Added.
(JSC::UStringImpl::baseSharedBuffer):
(JSC::UStringImpl::sharedBuffer):
(JSC::UStringImpl::destroy):
(JSC::UStringImpl::computeHash):
* runtime/UStringImpl.h: Added.
(JSC::UntypedPtrAndBitfield::UntypedPtrAndBitfield):
(JSC::UntypedPtrAndBitfield::asPtr):
(JSC::UntypedPtrAndBitfield::operator&=):
(JSC::UntypedPtrAndBitfield::operator|=):
(JSC::UntypedPtrAndBitfield::operator&):
(JSC::UStringImpl::create):
(JSC::UStringImpl::createCopying):
(JSC::UStringImpl::createUninitialized):
(JSC::UStringImpl::data):
(JSC::UStringImpl::size):
(JSC::UStringImpl::cost):
(JSC::UStringImpl::hash):
(JSC::UStringImpl::computedHash):
(JSC::UStringImpl::setHash):
(JSC::UStringImpl::identifierTable):
(JSC::UStringImpl::setIdentifierTable):
(JSC::UStringImpl::ref):
(JSC::UStringImpl::deref):
(JSC::UStringImpl::allocChars):
(JSC::UStringImpl::copyChars):
(JSC::UStringImpl::computeHash):
(JSC::UStringImpl::null):
(JSC::UStringImpl::empty):
(JSC::UStringImpl::checkConsistency):
(JSC::UStringImpl::):
(JSC::UStringImpl::UStringImpl):
(JSC::UStringImpl::operator new):
(JSC::UStringImpl::bufferOwnerString):
(JSC::UStringImpl::bufferOwnership):
(JSC::UStringImpl::isStatic):

JavaScriptGlue: 

* ForwardingHeaders/wtf/PossiblyNull.h: Added.
    - add forwarding header.

WebCore: 

* ForwardingHeaders/runtime/UStringImpl.h: Added.
    - add forwarding header.

* platform/text/StringImpl.cpp:
(WebCore::StringImpl::ustring):
    - order of arguments to UString::Rep constructor for shared strings changed.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@52463 268f45cc-cd09-0410-ab3c-d52691b4dbfc