Add benchmarks

[sorairolake]

Benchmark tests available in nightly Rust, and this can be run as follows:

cargo +nightly bench

If it's acceptable to add this, I'll write it for resvg and usvg.

• https://doc.rust-lang.org/unstable-book/library-features/test.html

[RazrFalcon]

You can use bencher on stable just fine.

As for the benchmarks, the main reason we don't have them is that they would be sort of pointless. We cannot easily compare resvg performance with other libraries and measuring resvg performance on its own doesn't really make much sense, since we're not changing anything, so regressions are unlikely.

What is your goal with benchmarks? And what exactly are you plan to benchmark?

[sorairolake]

What is your goal with benchmarks? And what exactly are you plan to benchmark?

Because I thought it would be useful to be able to measure the performance of resvg and usvg.

But if this doesn't make much sense, this addition may not be necessary.

[RazrFalcon]

Sorry, but no. Not very useful.