Update comment on variable expansion

I once thought that `Variable::expand` should require mutable self
because `Quirk::Random` would require mutable access to the random
number generator. However, mutable access to `Variable` depends on
`VariableSet::get_or_new`, which may bring on unexpected side effect of
creating a new variable when the variable does not exist.

Now I think that `Quirk::Random` should resort to internal mutability
with `RefCell` instead of requiring mutable access to `Variable`. This
commit updates the comment to reflect this change.

yash-env/src/variable/main.rs

@@ -141,7 +141,6 @@ impl Variable {
         self.read_only_location.is_some()
     }
 
-    // TODO Should require mutable self
     /// Returns the value of this variable, applying any quirk.
     ///
     /// If this variable has no [`Quirk`], this function just returns
@@ -151,6 +150,7 @@ impl Variable {
     /// This function requires the location of the parameter expanding this
     /// variable, so that `Quirk::LineNumber` can yield the line number of the
     /// location.
+    #[inline]
     pub fn expand(&self, location: &Location) -> Expansion {
         super::quirk::expand(self, location)
     }

yash-env/src/variable/quirk.rs

@@ -42,8 +42,8 @@ pub enum Quirk {
     /// the location of the parameter expansion. This `Quirk` is lost when an
     /// assignment sets a new value to the variable.
     LineNumber,
-    // TODO $RANDOM
-    // TODO $PATH
+    // TODO Random(RefCell<RandomState>)
+    // TODO Path(...)
 }
 
 /// Expanded value of a variable