BASIC main execution loop ignores rest of line after executing a single token

[gardners]

The basic main execution loop jumps to a statement's implementation, which then just jump to the main BASIC loop.

They should instead jump to a basic_next_statement routine, that checks if the input has been
consumed, and continues if there is still input.

It should also give SYNTAX ERROR if the next token is not a :, since that means there is cruft hanging around after the last statement.

[gardners]

Fixed.