Hi.

I've written a patch draft that replaces zlib with the zstd compression algorithm ([1])
in LTO. I'm also sending statistics that are collected for couple of quite big C++ source
files. Observation I did:

- LTO stream compression takes 3-4% of LGEN compile time
- zstd in default compression level (3) generated slighly smaller LTO elf files
- zstd compression is 4-8x faster
- decompression is quite negligible, but for a bigger project (godot) I can
  reduction from 1.37 to 0.53 seconds
- ZSTD API is much simpler to use

Suggestion based on the observation:
- I would suggest to make zstd optional (--enable-zstd) and one would
  use #include <zstd> + -lzstd
- I like the default level as we want to mainly speed up LTO compilation
- we can provide an option to control algorithm (-flto-compression-algorithm),
  similarly to -flto-compression-level
- we can discuss possible compression of LTO bytecode that is distributed between WPA
  stage and individual LTRANS phases.

Thoughts?
Thanks,
Martin

[1] https://github.com/facebook/zstd