Lexur, now with infinitely more digit errors

After looking at the example compilers, a lot of these errors are not
caught in lex, but I think they should be so they are. A digit/[a-z] is
never allowed after another digit so why bother allowing it in lex? At
present lexing stops if an error happens.

1. Error if digit followed by [a-z] (are you trying to start an id with
a digit?)
2. Error if digit followed by another digit (are you trying to have a
two digit number?)
3. Error if digit followed by an invalid character in the language (that
isn't allowed)
Otherwise we add the digit as a token.