Add function compilation

Functions are compiled in the most naiive way right now. I want to fix
up how scope lookups are done before it becomes too much to update.

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

src/compile/thunk.rs

@@ -345,9 +345,28 @@ impl Visit for CompileBody<'_> {
         Ok(thunk)
     }
 
-    fn visit_fun_expr(&mut self, _expr: &FunExpr) -> Self::Out {
-        // TODO : fun exprs should be compiled as constants and put into the constant pool
-        todo!()
+    fn visit_fun_expr(&mut self, expr: &FunExpr) -> Self::Out {
+        // - push const
+        // (functions are unique const values so a new function will be created for every literal
+        //  function defined in code)
+        
+        // This is pretty much the only place where a new scope layer gets pushed beyond the start
+        // of the program
+        self.compile.push_scope_layer();
+        for param in expr.params.iter() {
+            let sym = global_sym(param.to_string());
+            self.compile.create_local(sym);
+        }
+        let locals = self.compile.pop_scope_layer().unwrap();
+        let code = self.visit_body(&expr.body)?
+            .flatten()
+            .to_vec();
+        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)
+
+        // oh yeah, we were compiling a function body weren't we
+        Ok(Inst::PushConst(hdl).into())
     }
 
     fn visit_atom(&mut self, atom: &Atom) -> Self::Out {