Fix references to source code in compilation buffer for rustc 1.24

[Dushistov]

This change is based on #55
It uses cargo-process--custom-path-to-bin instead of hard coded
"cargo", plus it uses --manifest-path to run "cargo" against only one crate.

[Dushistov]

Known problem: if Cargo.toml mailformed, it reports JSON error, better will be report cargo stderr instead

[kwrooijen]

Would this PR completely invalidate #55 ?

Also I think the Cargo.toml issue is a bit trivial IMHO.

[Dushistov]

Would this PR completely invalidate #55 ?

Yes. #55 has issue - it re-builds the whole workspace, instead of current crate.
That was very inconvenient for me, and that's why I took #55 and redid.

Also I think the Cargo.toml issue is a bit trivial IMHO.

The current PR and #55 is pretty trivial, so feel free to close both PR and redo
this functionality from scratch without trivial to solve problems.

[kwrooijen]

I'll merge this and see if I have time to fix the Json issue. But really if it only occurs when you have malformed data I don't think it's a big issue. I'll close the other PR.

Thanks!