Restructure a bit

Rename function from compile to validate, add a new compile function
that first validates the program and then gets an Ast back from it.
Get rid of some unused args warnings
Write my own t_of_sexp function
Print out the parsed AST
Rename my sexp parsing function

Bring back the sexplib t_of_sexp function too.  Now we have a way to
serialize the already parsed ast of a postfix program.
Pretty print the AST