libtock-rs 2.0

[jrvanwhy]

This issue tracks the remaining work items for the Tock 2.0 libtock-rs crates. This is basically a list of things I have designed and prototyped, but not implemented.

• [x] Finish implementing the Allow system call in libtock_unittest
• [x] Design and implement the Allow API in libtock_platform
• [x] Add an Exit system call implementation to libtock_unittest
• [x] add unit tests for #333.
• [x] Design and implement the Exit syscall API in libtock_platform
• [x] Finish implementing Subscribe in libtock_unittest (#329, #330)
• [x] Design and implement the Subscribe API in libtock_platform
• [x] Implement a fake LowLevelDebug API. (#331)
• [x] Implement a LowLevelDebug driver.
• [x] Implement a #[panic_handler]
• [x] Implement a fake console API.
• [x] Implement a console driver.
• [x] Write a Hello World example.
• [ ] Implement a diagnostic for a missing stack_size! macro (and maybe a missing set_main! too).
• [ ] Document fake::Kernel (rough draft)
• [ ] Decide how we'll do integration testing (I think there's some sort of custom test framework unstable feature), and implement our integration testing framework.
• [ ] Update the test runner to work with the integration testing framework.
• [ ] Write a "how to add a driver to libtock_rs" document.