Improve unsafe casting

[mbarbin]

In #18 I made an attempt to make the unsafe casting part of the library more articulate, however I am still not happy with it.

I am not sure whether it is a guarantee that extensible variants with no argument be always physical equal. Also, there's nothing preventing the user from defining traits constructor with arguments, and I suspect things break down in the library in this case.

I'd be interested in getting ideas and opinions from other on this:

• Are there additional check that could be added to the library to help rejecting invalid usages?
• Are there some specific tests that could be added to make it more robust?
• Can we improve documentation and link relevant part of the runtime manual, etc?
• ?

[mbarbin]

After consider a few options, we think we have at least one viable option that will make the library type-safe without giving up on ergonomic aspects. Created milestone to track : https://github.com/mbarbin/provider/milestone/1

This work is done in collaboration with @v-gb.

[mbarbin]

This was included in the 0.0.11 release.