Remove ExecEngine abstraction

[matklad]

Hi! Not sure what was the idea behind exec engine (perhaps to allow swapping it out during tests?), but looks like it does absolutely nothing at the moment, and can be removed.

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

This was added way long ago in #1107 at the time to enable projects like cargo-emscripten. @tomaka is this still necessary or have those related projects since moved on?

[tomaka]

@alexcrichton I wrote the ExecEngine stuff in order to add flags when Cargo invokes Rust. In other words, exactly what cargo rustc now does.
So I guess it can be removed as it is superseded by cargo rustc.

[alexcrichton]

@bors: r+

Ok, thanks for the info @tomaka! @matklad there may be further cleanup from jettisoning this, haven't looked too closely yet though. If you find anything then feel free to take an axe to it!

[bors]

📌 Commit 9f6a8a7 has been approved by alexcrichton

[bors]

⌛ Testing commit 9f6a8a7 with merge 09a955c...

[bors]

☀️ Test successful - cargo-cross-linux, cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64
Approved by: alexcrichton
Pushing 09a955c to master...