JEP 401 spec updates

[Dan Smith]

I've gone through my list of spec feedback and tried to address all outstanding concerns. You can find the latest specs here:

https://cr.openjdk.org/~dlsmith/jep401/latest

See the change logs for details. The main new content here is specifying StackMapTables and verification rules for guaranteeing strict field initialization (revised a bit from the version I shared last week).

For convenience, I also created a redirect for the spec implemented by the EA release, which doesn't include the latest verification changes:

https://cr.openjdk.org/~dlsmith/jep401/ea