Perform benchmarks

[hecrj]

As mentioned in #4, in order to avoid performance regressions, it would be great to run some benchmarks on the different parts of the API.

The most expensive operation is clearly UserInterface::build. This will be the main focus for many of the optimizations we will implement in the future. We should benchmark this first.

UserInterface::update and UserInterface::draw will also get slower the more widgets and events a user interface has to handle. It would be great to have some numbers!

[tgotwig]

I can try to have a look 🕵🔎

[hecrj]

@tgotwig Awesome! I really appreciate the help. Let me know if you need anything.

[tgotwig]

CPU is constantly on 3.x on MacOS when I run it with --release, although nothing is happening.

[lachlansneff]

Looks to me like the event loop it's using is continuously polling.

[hecrj]

Yes, the example is drawing the UI every frame. This is expected.

Benchmarking the ggez example is not what this issue is about! iced is renderer-agnostic. We should benchmark iced.