Implemented one-less-car.
Implemented come-from-behind.
Defined map, filter, and cons.

Did not implement any of them yet.
Implemented map and filter.
Implemented cons.
Removed debug print statement.
Fixed a bug in lexer that didn't allow for empty lists.
Defined and implemented empty?
Did a few things to help debugging.

- Added print builtin function.
- Printed line numbers on error output.
- Also fixed how yknow works.