pkcs11 0.4.0

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# Rust PKCS#11 Library

This is a library which brings support for PKCS#11 to Rust. It is aiming at having both a very low-level API to map the PKCS#11 functionality to Rust as well as having a higher-level API for more easy usage as well as bringing more safety for programming against PKCS#11.

## Testing

Testing is currently done with [SoftHSM2](https://github.com/opendnssec/SoftHSMv2 "SoftHSM2 Repo"). A trillion thanks to the people at OpenDNSSEC for writing SoftHSM. This makes it possible to develop applications that need to support PKCS#11. I would have no idea what to do without it. (Suggestions are always welcome.)

### Status

Here is a list of the implementation status and plans on what to do next:
- [x] Dynamic loading of PKCS#11 module (thanks to [libloading](https://github.com/nagisa/rust_libloading "libloading Repo"))
- [x] Initializing and Dropping PKCS#11 context
- [x] Implementing Token and PIN Management functions
- [x] Implementing Session Management functions
- [x] Implementing Object Management functions
- [x] Implementing Key Management functions
- [x] Implementing Encryption/Decryption functions (TODO: tests still missing)
- [x] Implementing Message Digest functions (TODO: tests still missing)
- [x] Implementing Signing and MACing (TODO: tests still missing)
- [x] Implementing Verifying of signatures and MACs (TODO: tests still missing)
- [x] Implementing Dual-function cryptographic operations (TODO: tests still missing)
- [x] Implementing Legacy PKCS#11 functions
- [x] Reorganize code of low-level API (too bloated, which we all know is what PKCS#11 is like)
- [x] Import the rest of the C header `pkcs11t.h` types into rust
- [x] Import the rest of the C header `pkcs11f.h` functions into rust
- [ ] C type constants to string converter functions, and the reverse (maybe part of the high-level API?)
- [ ] Design and implement high-level API
- [x] Publish on crates.io (wow, that was easy)
- [ ] Write and Generate Documentation for Rust docs
- [ ] Better Testing (lots of repetitive code + we need a testing framework and different SoftHSM versions for different platforms)