[ffigen] Benchmarks running FFIgen

We have performance concerns for ffigen when run on large ObjC APIs, as the bindings can be >100k lines. We should add a benchmark that runs ffigen on one of the Apple frameworks and measures:

- Clang parse time
- Binding generation time (without formatting)
- Formatting time
- Size of the generated code (in lines and bytes)
- Max memory use

The benchmark should also run the analyzer over the generated code and measure:

- Analysis time
- Max memory use

This benchmark is easy to write. The part I'm not sure about is how to integrate it into github CI. Is there a good framework for continuous benchmarking? Or should we just run the benchmark script locally as needed?