New version of the ISO8583 package.

Here are some goals we wanted to achieve.

• It is more explicit with spec definitions and uses Go types instead of text or tags:
  • provides type checking
  • code completion
  • better readability
• It uses interfaces for low-level building blocks like encoding, padding, etc.:
• It makes it possible to add new implementations of encodings, etc., easily.
• It supports composable fields and higher-level abstractions like bitmaps, date fields, amount fields, etc.