Lexer parser

Implementation of a lexer and parser for System F. For the moment, parse the entire file and evaluate/type-check the expression. Improvement/TODO:

• Allow to use ;; to end an expression => it allows to write multiple expressions in the same file.
• Remove useless comments.
• Undo changes about printing characters (like λ, Λ and ∀). It was only for ease of reading. It implies to change the symbol in the lexer.