extendr-impl: Return the right reference

[CGMossa]

Following the polishing of ExternalPtr is this PR on returning the right reference.

First, we remove the ability to take &T and make ExternalPtr<T>. This is a flawed construct: Making an ExternalPtr is not just wrapping a pointer, it is also signing a pointer up for destruction. Converting a &T to ExternalPtr<T> may cause spurious double-free events.

The Rules of ExternalPtr are simple:

• From an owned type, one may create an Owned Pointer, and from an owned pointer, one can instantiate a true ExternalPtr.
• From &ExternalPtr<T> you can create &T, and similarly for &mut ExternalPtr<T> and &mut T.

So, should there be no references used or returned? A reference that comes from a true ExternalPtr<T> may be used. We embed this logic on the wrapping function, rather than do any C-crimes.

If a reference corresponds to any of the ExternalPtr<T> in the argument list, then that is returned.

Fixes #725

[CGMossa]

At this stage, I now check the address of the returned-object, and see if it matches the ExternalPtr from Self.

Error (test-test-submodules.R:32:3): Return the right externalptr
Error in `x$max_ref(y)`: only able to return reference to self
Backtrace:
    ▆
 1. └─x$max_ref(y) at test-test-submodules.R:32:3

Next step is to go through all arguments, that are externalptr, and return the one that maches the output, or error otherwise.

Thankfully, it is impossible to create a local variable and return a reference from it in Rust.

[CGMossa]

This is ready for feedback, not necessarily review.