Add benchmarks for push, insert, extend, and pushpop

[nipunn1313]

Want to add these benchmarks so that we can more effectively detect regressions/improvements from PR's like #28 and #29.

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This change is

[emilio]

This won't compile in stable, will it?

[nipunn1313]

Correct. Let me work on factoring it into a conditionally compiled feature which is disabled by default.

[nipunn1313]

Looks like this on my machine (Mac book pro laptop)

Nipunns-MacBook-Pro:rust-smallvec nipunn$ cargo bench --verbose --features=benchmarks bench
Fresh smallvec v0.2.0 (file:///Users/nipunn/src/rust-smallvec)
Finished release [optimized] target(s) in 0.0 secs
Running `/Users/nipunn/src/rust-smallvec/target/release/deps/smallvec-a59689f47ae774cb bench --bench`
running 4 tests
test bench::bench_extend ... bench: 169 ns/iter (+/- 19)
test bench::bench_insert ... bench: 1,201 ns/iter (+/- 267)
test bench::bench_push ... bench: 295 ns/iter (+/- 39)
test bench::bench_pushpop ... bench: 357 ns/iter (+/- 117)
test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured

Travis CI looked like this. I don't think we can really rely on its numbers though.

running 4 tests
test bench::bench_extend ... bench: 182 ns/iter (+/- 3)
test bench::bench_insert ... bench: 1,916 ns/iter (+/- 21)
test bench::bench_push ... bench: 307 ns/iter (+/- 6)
test bench::bench_pushpop ... bench: 383 ns/iter (+/- 6)```

[nipunn1313]

Any update on this?

[jdm]

@bors-servo: r+
Thanks for doing this work!

[bors-servo]

📌 Commit 951f1d0 has been approved by jdm

[bors-servo]

⚡ Test exempted - status