Parser ignores lack of whitespace between integers and identifiers

[ashn-dot-dev]

$ ./minilisp 
(define a 1)
1
(+ 2a)
3

I noticed that read_expr only checks if the current character being parsed is a digit before invoking make_int.
Perhaps not worth fixing for at this point, but may be useful to know for anyone else using this as learning resource for their own interpreter implementation.