[umbrella task] Track and improve code size

[terrelln]

Zstd has been getting larger over the years, as we've added features and improved performance. We should take a comprehensive look at code size, and try to optimize our code size while retaining performance.

1. Explicitly track our code size in our tests, and monitor between releases.
2. Look at the largest code size files and try to find wins. a. zstd_opt.c is a prime target: Issue #2866. b. zstd_lazy.c was optimized in PR #2828, but there is still more that can be won (as mentioned in the TODO).
3. Enable modular builds. E.g. allow cutting out dictionary support, or cutting out high compression levels.

[sav-ix]

hello,

wondering, if dropping build flags because of buggy compiler is a standard way to solve the issues. what will happen, if -O1 became buggy on that parisc system? if something different, than in e2d01863bce98a4642c98cf7727ed8bfb0f33c81, why this something didn't happen for -O3 bug? thanks.