feat: get map diff

[uptutu]

Diff get a map record the difference between the two maps.

The calculation is based on a map change to b map
if deleted some field in a map then the field will be returned in result field same key but is nil value.

 a map {"a":"a","b":"b"}
 b map {"b":"b","c":"c"}
 return map {"a":nil,"c":"c"}

close: #48

[uptutu]

#48