json example:
  check for and report control characters during parsing;
  otherwise, error reports can be off
json example: since parser is now responsible for checking for control characters in string literals, remove check from jsontree
json example: move checking of leading 0s and valid escape sequences to parser

justification:
 - standard conformant
 - better error messages in case of a mis-parse
 - simpler
json example: strings can no longer cause errors in AST construction
  phase, so take advantage of that to simplify