Unsupported types should be poisoned similarly to the stdlib

Consider https://godbolt.org/z/5W1x94aTh

We see that:
```cpp
static_assert(std::is_invocable_v<std::hash<rawr>, rawr>);
```
correctly fails until
```cpp
template<>
struct std::hash<rawr> {
  std::size_t operator()(rawr const&) const noexcept { return 0; }
};
```
is defined.

This would enable downstream consumers to write simple `is_hashable` trait checks.