Got (tail) recursive functions working (woo!) mutuialy (tail) recursive
functions work too, by concidence.

I also added some stuff to the regression tests

And fixed up a little issue it the frontend, eof on a line after an open
paren(i.e a line started with >) will quit upto the top level rather
than exiting the program

Lastly added functions to test for even/oddness and if an number is zero