Finish up function call implementation, it appears to be working

Functions are downcasted to a `Fun` trait, which will construct the
appropriate stack frame.

A few other things have been shifted around that affect internal APIs
while things are still under construction.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/thunk.rs

@@ -382,9 +382,13 @@ impl Visit for CompileBody<'_> {
             })
             .collect();
 
-        let code = self.visit_body(&expr.body)?
+        let mut code = self.visit_body(&expr.body)?
             .flatten()
             .to_vec();
+        // XXX TODO(compile)
+        // remove this when we get returns implemented
+        code.push(Inst::PushSym(crate::obj::reserved::NIL_NAME.sym));
+        code.push(Inst::Return);
         let (hdl, _fun) = self.compile.push_const(UserFun::new_obj(code, locals));
 
         // TODO(compile) : determine return value at the end of the body (preferably at parse-time)
@@ -397,6 +401,7 @@ impl Visit for CompileBody<'_> {
         let thunk = match atom {
             Atom::Ident(ident) => {
                 let sym = global_sym(ident.to_string());
+                // TODO : use "LOAD_GLOBAL" instead of "LOAD_LOCAL" when inside a user function
                 if let Some(local) = self.compile.lookup_local(sym) {
                     // get local
                     Inst::LoadLocal(local).into()