r2d2 adaptor for cryptoki

Session pool manager for cryptoki.

Example

```rust norun use r2d2cryptoki::; use cryptoki::context::;

let mut pkcs11 = Pkcs11::new("libsofthsm2.so").unwrap(); pkcs11.initialize(CInitializeArgs::OsThreads).unwrap(); let slots = pkcs11.getslotswithtoken().unwrap(); let slot = slots.first().unwrap(); let manager = SessionManager::new(pkcs11, *slot, SessionType::RwUser(Pin::new("fedcba".tostring())));

let pool = r2d2::Pool::builder().build(manager).unwrap();

let session = pool.get().unwrap(); println!("{:?}", session.getsessioninfo().unwrap()); ```