Rustup tight integration

[KalitaAlexey]

Currently there are the following opened issues: #242, #238, #231, #225, #187.
I am going to reform an approach I have chosen.

At the first iteration, I am going to remove updating rustup since it is useless most time.

At the second iteration, I am going to reform how the extension checks if some path belongs to rustup.
Currently it checks if the result of rustc --print=sysroot contains ".rustup". I made the wrong assumption because some people has it differently.
It will use rustup run nightly rustc --print=sysroot.
At this point those who don't use the nightly toolchain will show an error message saying that the nightly toolchain is not installed.
It will check with rustup component list --toolchain nightly.
It will install with rustup component install rls --toolchain nightly.

At the third iteration, I am going to make the extension install the nightly toolchain.
At this point users who have Rustup will no longer have any issues.

At the fourth iteration, I am going to make the extension install Rustup.

At the fifth iteration, I am going to make the extension offer a user to choose which way one wants the extension to run.

Let me know if you want something to be done differently.

[wagenet]

@KalitaAlexey the issue for me was that I had the PATH added in my ~/.zshrc not my ~/.profile. When I moved it, then it started working. I may be mistaken, but it's not clear to me that the changes you detailed here would have solved the problem for me.

[KalitaAlexey]

@wagenet,
If the extension couldn't find rustup, then it wouldn't work.
This thing will not be changed.

[wagenet]

Fair enough :) I suspect setups like mine are not rare, so it might make sense to provide some troubleshooting information somewhere. The fix is easy, just not immediately obvious.

[KalitaAlexey]

@wagenet,
I will think about it.