Add predicate to substitute functions

```haskell
substitute :: Monad f => Eq a => a -> f a -> f a -> f a
substitute = substituteOf . (==)

substituteBy :: Monad m => (a -> Bool) -> m a -> m a -> m a
substituteBy old new as = do
a <- as
if old a
then new
else pure a
```

```haskell
substituteVar :: Functor f => Eq a => a -> a -> f a -> f a
substituteVar = substituteVarOf . (==)

substituteVarBy :: Functor f => (a -> Bool) -> a -> f a -> f a
substituteVarBy old new as = do
a <- as
pure
if old a
then new
else a
```

Is there a use for this?