Improved ergonomics for dynamic message

[andrewhickman]

• Implement PartialEq on Value with primitive types to allow things like message.get_field_by_name("foo") == 5
• Implement Index and IndexMut with string and numeric keys to allow message["foo"] = Value::I32(5)
• Some way to get a value from multiple levels deep into the message? e.g. message.get_nested("foo.bar[0]")