./miri improvements
I have needed to run something with many different seeds often enough that I would like an easier way to do it. ;) So now we have `./miri many-seeds`.
Also I made the script less dependent on the working directory, so calling it from a different directory should work properly now even if that other directory does not have the same rustup override as the one where Miri lives.
Support no-std targets and test it in CI
cc `@jamesmunns`
This is a bit annoying as you need to have `MIRI_NO_STD=1` set at all times, but it works ™️
Once libstd's `restricted_std` feature becomes more usable, we can probably do away with that env var.
I also added a test to CI to make sure it keeps working. This test only builds libcore and runs a single test, so it's pretty fast.
revert --color=always changes
They [cause problems](https://github.com/rust-lang/miri/issues/2277) and they completely break rendering on the playground:
```
Compiling playground v0.0.1 (/playground)
Finished dev [unoptimized + debuginfo] target(s) in 0.47s
Running `/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri target/miri/x86_64-unknown-linux-gnu/debug/playground`
[0m[1m[38;5;9merror[E0277][0m[0m[1m: `*mut std::sync::atomic::AtomicUsize` cannot be sent between threads safely[0m
[0m [0m[0m[1m[38;5;12m--> [0m[0msrc/main.rs:17:18[0m
[0m [0m[0m[1m[38;5;12m|[0m
[0m[1m[38;5;12m17[0m[0m [0m[0m[1m[38;5;12m|[0m[0m [0m[0m let j1 = spawn(move || {[0m
[0m [0m[0m[1m[38;5;12m| [0m[0m [0m[0m[1m[38;5;12m__________________[0m[0m[1m[38;5;9m^^^^^[0m[0m[1m[38;5;12m_-[0m
[0m [0m[0m[1m[38;5;12m| [0m[0m[1m[38;5;12m|[0m[0m [0m[0m[1m[38;5;9m|[0m
[0m [0m[0m[1m[38;5;12m| [0m[0m[1m[38;5;12m|[0m[0m [0m[0m[1m[38;5;9m`*mut std::sync::atomic::AtomicUsize` cannot be sent between threads safely[0m
[0m[1m[38;5;12m18[0m[0m [0m[0m[1m[38;5;12m|[0m[0m [0m[0m[1m[38;5;12m|[0m[0m [0m[0m *(c.0 as *mut usize) = 32;[0m
[0m[1m[38;5;12m19[0m[0m [0m[0m[1m[38;5;12m|[0m[0m [0m[0m[1m[38;5;12m|[0m[0m [0m[0m });[0m
[0m [0m[0m[1m[38;5;12m| [0m[0m[1m[38;5;12m|_________-[0m[0m [0m[0m[1m[38;5;12mwithin this `[closure@src/main.rs:17:24: 19:10]`[0m
[0m [0m[0m[1m[38;5;12m|[0m
```
Sorry `@saethlin,` I think we need to go back to start here and consider another solution.
Fixes https://github.com/rust-lang/miri/issues/2277
