### Numbers ###

Change the lexer such that leading 0 is not part of a number:

    # current behaviour
    01     # -> 1
   
    # new
    01     # -> 0 1

Also negative numbers should be written with `_` instead:

    # current behaviour
    1 2-3   # -> -1 3
   
    # new
    1 2_3     # -> 1 2 -3