Implement basic support for concurrency (Linux/macos only)
Changes (most new code is in `src/threads.rs` and `src/shims/foreign_items/posix.rs`):
1. Move the stack from `Machine` to a newly created `Thread` struct.
2. Add a `ThreadSet` struct that manages the threads.
3. Change `canonical_alloc_id` to create a unique allocation id for each thread local and thread (the responsible struct is `ThreadLocalStorage`)
4. Change the code to execute the thread local destructors immediately when a thread terminates.
5. Add the most basic round-robin scheduler.
This pull request depends on [these changes to the compiler](https://github.com/rust-lang/rust/pull/70598).
Details to the found in https://github.com/rust-lang/miri/issues/1364.
Note that this was not a found in a `master` or production release of ink!, however without analysing the code via `miri` this could have potentially happened.
This updates the installation instructions to point out that miri is only available on nightly, and encodes this assumption into the installation instructions.
Add a scheme to find the place where an id was destroyed
cc https://github.com/rust-lang/miri/issues/974
I'm not too happy with it, but since stacked borrows don't have access to the current call stack, I can't just report a warning as per https://github.com/rust-lang/miri/issues/797
We could add some global mutex that we can throw strings at and `step` will clear out that mutex and report warnings before moving the `statement_id` or the `block_id`, not sure how well that would work. For now I think this is sufficient
