with: add sfinae for nicer compile failure messages

Had we not been operating in the context of a CRTP, the sfinae would
have been:

```
template <typename T,
  std::enable_if_t<
    std::is_same_v<
      T,
      typename decltype(std::decval<Deriv>().make())::value_type
    >
  , int> = 0
>
```

However, this cannot work, for then compilers will complain that we're
referencing an incomplete type in the form of std::declval<Deriv>.

We need to delay the type instantiation with the following form:

```
template <typename T,
  typename U = Deriv,
  std::enable_if_t<
    std::is_same_v<
      T,
      typename decltype(std::decval<U>().make())::value_type
    >
  , int> = 0
>
```

As far as I can tell, it's essentially this problem:

https://stackoverflow.com/questions/43307462

lager/with.hpp

@@ -172,7 +172,14 @@ class with_expr_base : public xform_mixin<Deriv>
         return deriv_().zoom(lens);
     }
 
-    template <typename T>
+    template <typename T, 
+             typename U = Deriv,
+             std::enable_if_t<
+                  std::is_same_v<
+                    typename decltype(std::declval<U>().make())::value_type,
+                    T
+                  >,
+               int> = 0>
     operator reader<T>() &&
     {
         return std::move(*this).make();
@@ -182,7 +189,14 @@ class with_expr_base : public xform_mixin<Deriv>
     {
         return std::move(*this).make();
     }
-    template <typename T>
+    template <typename T, 
+             typename U = Deriv,
+             std::enable_if_t<
+                  std::is_same_v<
+                    typename decltype(std::declval<U>().make())::value_type,
+                    T
+                  >,
+               int> = 0>
     operator cursor<T>() &&
     {
         return std::move(*this).make();