implement --optimize-font-speed option for larger/faster glyf/gvar like fonttools'

@behdad added a compile option in fonttools for glyf and gvar tables that enables some optimizations that prioritize speed over file size. Maybe we could implement the same in fontc and write-fonts, as an option.

https://github.com/fonttools/fonttools/pull/3784

I don't exactly know what the actual performance improvements may be, it'd be interesting to get more data on that; however, I noticed that after woff2-compressing a larger speed-optimized version of Google Sans Flex, it actually ends up 40kb smaller than woff2-compressed regular (size-optimized) font.

Start by reading the linked fonttools pull request and locating the compile-option entry points in fontc and write-fonts. The work is done when both tools expose the optimization option for glyf and gvar tables and performance and compressed-size results are available for comparison.

Written by the indexing model from the issue text.