Allow usage of ?Sized types in register_shared

[nicopap]

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[nicopap]

Hmm. I think I've a workaround for what I thought I needed this for. Instead of storing directly Arc<T> for your non-sized T, store Arc<Box<T>>.

To make it short, register_shared accepts a Arc<T> as return type of the factory. But T: Sized, which can be problematic if we want to return T = dyn Trait.

The reason T: Sized is that *const T for T: !Sized is a fat pointer, and rust doesn't expose ways to manipulate metadata in fat pointers, generally it's impossible to just store a *const T for unsized type and get back a &T.

Generally, because with the hack I introduce, it's technically possible.