Fix several issues in System.Json

Fix several issues in System.Json

- Fix detection of extra characters in JSON input
- Fix detection of invalid negative numeric literals
- Disallow leading zeros
- Use "round-trip" format for floating point numbers
- Serialize NaN and Infinity as strings
- Use NumberFormatInfo.InvariantInfo for Convert.To*()
- Use standard number parsing methods in ReadNumericLiteral()
- Add test cases