Make eval a decorator

`eval` has always been implemented as a function, which was always a bit
of a hack that caused some issues such as triggering the creation of a
new scope. This turns `eval` into a decorator.

The scoping issues with eval prevented it from being usable to actually
implement other shell components in fish script, such as the problems
described in #4442, which should now no longer be the case.

Closes #4443.

src/complete.cpp

@@ -1469,8 +1469,8 @@ void completer_t::perform() {
                 use_command = true;
                 use_function = true;
                 use_builtin = true;
-                use_implicit_cd = false;
-                use_abbr = false;
+                use_implicit_cd = true;
+                use_abbr = true;
                 break;
             }
         }