SpiderMonkey 78

Now with FireFox 78 ESR released, we should upgrade to SpiderMonkey 78.

Steps, roughly in order:

• To make optimal use of help and information from Mozilla, port https://github.com/spidermonkey-embedders/spidermonkey-embedding-examples/ to SpiderMonkey 78 first ➡ https://github.com/mozilla-spidermonkey/spidermonkey-embedding-examples/pull/31
  • Write a migration guide while doing it
  • Get SpiderMonkey team members to review the port and the migration guide, and learn from them
• Port GJS and make sure all the tests pass ➡ !458 (merged)
• Make sure all the comments and documentation are up to date, git grep 68 will usually reveal some stragglers
• git grep COMPAT: for any comments about code that is waiting on SpiderMonkey bugfixes
• Fix any bugs in SpiderMonkey that are revealed by porting
  • https://bugzilla.mozilla.org/show_bug.cgi?id=1654696
  • error: options `-C embed-bitcode=no` and `-C lto` are incompatible
• Upload a definitive SpiderMonkey 78 source tarball to http://ftp.acc.umu.se/pub/GNOME/teams/releng/tarballs-needing-help/mozjs/
• Build mozjs78 images in GJS's CI
• Add mozjs78 module to the GNOME SDK
• Add mozjs78 module to JHBuild ➡ jhbuild!97 (merged)
• Write release notes, which can be compiled from the Firefox developer release notes of versions 69 through 78: for example, https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/69#JavaScript