Updated nestmates spec

[Dan Smith]

I've posted an update to the nestmates spec based on discussions of the last few months.

http://cr.openjdk.java.net/~dlsmith/nestmates.html

Significant changes:
- Renamed MemberOfNest --> NestHost, per some requests; it seems more consistent with names of other attributes
- Eliminated checking of the NestHost attribute during verification. This means the nest of a class is computed lazily when access is requested, and we have to account for live classes whose NestHost attribute is invalid

I considered and rejected a proposal to guard 'invokeinterface' behavior changes on the class file version number. Consensus was that behavioral changes that replace errors with successful execution are acceptable when running old class files.

Feedback is always welcome.

—Dan