https://bugs.webkit.org/show_bug.cgi?id=67340

In preparation to allowing StringImpl to be backed by 8 bit 
characters when appropriate, we need to eliminate or change the
usage of StringImpl::characters().  Most of the changes below
change s->characters()[0] to s[0].

Reviewed by Geoffrey Garen.

* bytecompiler/BytecodeGenerator.cpp:
(JSC::keyForCharacterSwitch):
* bytecompiler/NodesCodegen.cpp:
(JSC::processClauseList):
* interpreter/Interpreter.cpp:
(JSC::Interpreter::privateExecute):
* jit/JITStubs.cpp:
(JSC::DEFINE_STUB_FUNCTION):
* runtime/Identifier.cpp:
(JSC::Identifier::addSlowCase):
* runtime/JSGlobalObjectFunctions.cpp:
(JSC::jsToNumber):
(JSC::parseFloat):
* runtime/JSString.cpp:
(JSC::JSString::substringFromRope):
* runtime/JSString.h:
(JSC::jsSingleCharacterSubstring):
(JSC::jsString):
(JSC::jsSubstring):
(JSC::jsOwnedString):
* runtime/RegExp.cpp:
(JSC::regExpFlags):
* wtf/text/StringBuilder.h:
(WTF::StringBuilder::operator[]):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@94336 268f45cc-cd09-0410-ab3c-d52691b4dbfc