Allow for move-only components.

Right now, trying to do `e.assign(...)` for some C that has a deleted copy constructor will cause a substitution error when instantiating ComponentHelper::copy_component_to, even if that method is never called as the vtable still must be constructed. It would be nice to have proper support for move-only components as well.

A simple solution is to use SFINAE on ComponentHelper:
```
template
+class ComponentHelper>>: public BaseComponentHelper { ... }
```
and to not actually copy in copy_component_to, but that delays any errors trying to copy until runtime. That is suboptimal, and there should be a much more elegant solution.