This is the main function implementing the
read is immediately called from within the REPL. Given a string (comming from the programming
str, it initializes the scanner with the string, and
initializes the global variable
token so that the parser's 'current'
is 'ready' for processing. Hence,
token is always one token ahead
of the parser. After the first token has been determined,
is immediately called since we know from the concrete
grammar that any Pico program is always an expression.