hashers: convert illegal contraint to static_assert

The constraint on on cryptopp_hasher<>::impl is illegal, since it's not
on the base template. Convert it to a static_assert.

We could have moved it to the base template, but that would have undone
the work to push all the implementation details in .cc and reduce #include
load.

Found by clang.

hashers.cc

@@ -38,8 +38,8 @@ concept HashUpdater =
     };
 
 template <typename T, size_t size>
-requires HashUpdater<T>
 struct cryptopp_hasher<T, size>::impl {
+    static_assert(HashUpdater<T>);
     using impl_type = typename hasher_traits<T>::impl_type;
 
     impl_type hash{};