Wire up more builtins and slightly improve printing

src/builtins.c

@@ -8,10 +8,6 @@
 #include <string.h>
 #include <stdlib.h>
 
-LISP_FUNC_0(ucl_builtin_hello_world) {
-    return ucl_string_create(strdup("Hello, world!"));
-}
-
 LISP_FUNC_1(ucl_builtin_type, arg) {
     switch (arg->type) {
         case UCL_TYPE_CELL: