Call write_fmt directly to format an Arguments value.

[rprichard]

It's just as convenient, but it's much faster. Using write! requires an
extra call to fmt::write and a extra dynamically dispatched call to
Arguments' Display format function.

[rust-highfive]

r? @brson

(rust_highfive has picked a reviewer for you, use r? to override)

[rprichard]

r? @alexcrichton

I benchmarked format!("Hello World") and saw a reduction from 77 ns/iter to 54 ns/iter:

// ORIGINAL CODE:
// test simple_format ... bench:        77 ns/iter (+/- 2)

// MODIFIED CODE:
// test simple_format ... bench:        54 ns/iter (+/- 6)

#![feature(test)]
extern crate test;

#[bench]
fn simple_format(b: &mut test::Bencher) {
    b.iter(|| {
        test::black_box(format!("Hello World"));
    });
}

[alexcrichton]

@bors: r+ 317eac3

Nice wins!

[bors]

⌛ Testing commit 317eac3 with merge fcf637b...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt