Consider implementing profile-guided optimization

[shssoichiro]

Rust will in their next stable release enable support for profile-guided optimization. Other encoders such as x264 allow this as a technique to make the compiled binary faster. We could investigate using it as well.

[shssoichiro]

I took a shot at this, profiling with a variety of speed settings, quantizer, bitrate, etc similar to how x264 does it. The PGO version ended up being about 7% slower, and also took over a half hour to profile (that part was expected, though). So... This may be a bust, unless someone knows something I'm missing.

[jrmuizel]

This seems to be a known issue: http://lists.llvm.org/pipermail/llvm-dev/2019-September/135078.html
@michaelwoerister