CRDB fork of pkg/json

These files are a fork of pkg/json package.

• scanner.go, decoder.go, and reader.go were copied from pkg/json
• Additional utility methods added (e.g. to reset decoder)
• Ability to query reader position, etc.
• Package name renamed from json to tokenizer
• Removal of unused functions
• Probably, the largest change from the original is in the scanner.go. Changes were made to support un-escaping escaped characters, including unicode characters.