Introduce lib finalize method

[bukka]

This is to allow explicit calling of C_Finalize.

My use case is a test for my fork of pkcs11-proxy where I added tests which use this project. The way how pkcs11-proxy works is that it creates a daemon which gets the proxied calls. The issue that I have is that in my test I can start the daemon but then I need to also terminate it. However the termination should happen after getting C_Finalize which is difficult to do in pkcs11-proxy because it happens at the end when the garbage is collected. So I would like to do an explicit call to do finalize and then terminate the daemon.

[mathiasertl]

LGTM, but we should also test and document this function.

Btw. It might also solve an issue I encountered with the unit tests for my own project.

[bukka]

Cool. I will try to put together some test in the coming weeks and also update the docs.

[mathiasertl]

Thanks! Otherwise I see no reason against a merge!