v0.2.0: Improve lexer flexibility and add ergonomic parsers

[jangko]

• Put non-standard features behind flags.
• Add limit features to improve security.
• Add a rich set of parser to help users easily and safely crafting custom parser. No need to access unsafe lexer anymore.
• Improve documentation a lot.
• Add fuzz test to ensure json-serialization will not crash given malicious inputs.
• Add a lot of tests, locally or from Json test suite.
• Conform to the RFC8259 spec.
• The old lexer is gone, so it will break code which using lexer directly.

fix #30
fix #27
fix #22
fix #38
fix #37