Replace try!() with ?

[kornelski]

No description provided.

[TedDriggs]

I believe there were performance issues with this in the past; I asked @dtolnay about why serde used try! a while back and there were good reasons at the time.

[dtolnay]

Yes it measurably hurt our benchmarks -- tracked in rust-lang/rust#37939. I would imagine it matters less for Racer.

[kornelski]

Oh wow. It does compile to slightly worse code indeed.

Fortunately in Racer there weren't any try!s in hot loops.

[nrc]

Thanks!