add basic sexp type and functions
add Len() to Sexps
add helper function for casting Sexps

We want to be able to count nil as a legal list
overhaul golftalk to use real s-expressions for lists!
Merge branch 'sexp' of https://github.com/ComputerDruid/golftalk into ComputerDruid-sexp

Conflicts:
	builtin.go