Fixes #805

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=6022

Context needs to be called 'c'. The TRACE_* rely on that.

Previously adjusted logic could fail if nothing around was ligated,
and something moved.  This should take care of that.

CC https://github.com/harfbuzz/harfbuzz/issues/740

Add a continuous builder using fswatch

I hope I am in a correct direction at least

Optimize set through optimized bit operations

Should fix Win64 bot.

Fixes #791.

Should bring bag djgpp bot.

[subset] Use gids_to_retain_sorted to produce old gid -> new gid mapping since it now has the more complete set.

I'm confident that all bugs are hashed out now.