Simplify badToken in FusedBlockJsonEventIterator

A little bit extra DRY
Identifiers are started by UnicodeIdentifierStarts, not JavaIdentifierStarts
Use skipChar instead of nextChar in appropriate places

  * skipping to end of line
  * skipping closing quotes of strings
  * skipping '+' sign in numeric exponents
Add missing parens on a 0-args method call

Just a style thing
Make all the fields of FusedBlockJsonEventIterator private[this]

To avoid the private accessors