Cleanup of ScannerConstants, now named UnicodeCache.

Cleanup of ScannerConstants, now named UnicodeCache.

The ScannerConstants class was originally static fields on the scanner class.
During creation of the stand-alone preparser and later isolates, it has been
moved into a separate class with a per-isolate instance.
It is used to hold caching unicode Predicate values.

This change renames the class to UnicodeCache, and passes a reference
to the instance down to methods that doesn't have an easy access to
an isolate (to avoid, e.g., having to do an Isolate::Current() for every
number parsed).

Review URL: http://codereview.chromium.org/6824071

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7584 ce2b1a6d-e550-0410-aec6-3dcde31c8c00