FIX XML validation

[ldanko]

• [ ] check if MsgType variants match with defined messages
• [ ] check if there is no duplicated fields in messages
• [ ] check if fields/variants/types names can be valid rust identifiers (ex. number at the beginning)
• [ ] check if Header/Trailer is defined and if crucial fields are defined and have valid tag numbers (like BeginString, BodyLength, Checksum, MsgType, SeqNum, etc)