cat: Improve performance on Linux

[ArniDagur]

This PR should improve cat performance, especially on Linux. It does this by:

• Using trait monomorphization rather than trait objects (Box<dyn Trait>).
• Using splice on Linux

This PR supercedes #1289

Unscientific benchmarks

GNU cat

[arni][~/src/coreutils][use-splice2]% cat /tmp/bigfile | pv -r > /dev/null
[4.47GiB/s]
[arni][~/src/coreutils][use-splice2]% cat /tmp/bigfile | pv -r > /dev/null
[4.68GiB/s]
[arni][~/src/coreutils][use-splice2]% cat /tmp/bigfile | pv -r > /dev/null
[4.65GiB/s]

Before removing trait box, without Linux specialization

[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[3.41GiB/s]
[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[3.40GiB/s]
[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[3.52GiB/s]

After removing trait box, without Linux specialization

This is the figure you should expect for non Linux platforms.

[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[7.76GiB/s]
[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[5.67GiB/s]
[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[6.61GiB/s]

Linux specialization

[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[29.6GiB/s]
--- 22:33:36 ---
[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[14.4GiB/s]
--- 22:33:37 ---
[arni][~/src/coreutils][use-splice2]% target/release/cat /tmp/bigfile | pv -r > /dev/null
[22.2GiB/s]

[Arcterus]

Looks pretty good. In the future, I think we'll want to extract the splice() bits into a function or a type that serves as both a reader and a writer and throw it into uucore, but that can be done later.