The current json parser and scanner inherits fromt he normal scanners and parsers,
which are more complicated than we need for parsing json.

The supplied scanner works directly on the string supplied and has a
fast case mode for scanning only ascii characters (it will simply
create a substring or a symbol directly from the existing string). To
allow for creating symbols from a substring I have added a
SubStringAsciiSymbolKey that creates the hash based from our string
without extracting the sub-string. In case we need to add the symbol
it simply creates the symbol directly from the characters inside the
given string.
Review URL: http://codereview.chromium.org/7039037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8029 ce2b1a6d-e550-0410-aec6-3dcde31c8c00