Provide option to use non-exclusive references for all subclass calls

[adetaylor]

To allow re-entrancy even in the case that a C++ virtual function call is non-const yet re-entrant.

[adetaylor]

Related: dtolnay/cxx#936. (The reason cxx would not want this, but we might, is explained well in dtolnay/cxx#936 (comment))

[ericonr]

I was hit by this issue when trying to inherit from asynPortDriver; almost every function is virtual (probably without need), so it's really easy to hit a runtime panic. While I was able to get a minimal example running, it was severely limited.

Since upstream is open source, I will try to suggest changing the class definition to decrease the amount of virtual functions (I think a lot of them have no reason to be overridden). However, for now, the re-entrancy limitation of autocxx has been our main road-block.

That said, this has instigated us in pursuing other alternatives for integration, and we might do something else, especially in the interest of exposing safe Rust interfaces.