Error handling

[benoitlx]

I'm trying to do too much with these changes. This is why it's only a draft.
A rewrite will be completed in this branch

Problematic elements

• Trying to do the job of a parser
  • comparing labels in order to prevent multiple definition
  • aggregating identifier and value for the DEFINE variant
• Error management is painful : lot of boilerplate in lexer_error.rs
  • need to use macro in order to reduce boilerplate
  • need to flatten enums instead of separate struct
• Callback functions for Token should be within impl Token {}
• There should be less variant for Token (e.g. one Register(Reg) variant instead of A, V, ...)