Problem: json.Number will marshal to number but not string

[calvinaco]

json.Number has the following limitations:

1. It does not support uint64
2. It always marshal to number, which may creates problem when being parsed in some langauges

For better handling of number across different languages, add a new json struct to support

• Unmarshal from both string and numeric representation of the underlying number
• Marshal to number and string based on the range of the number

[calvinaco]

cc @davcrypto