Value class spec updates

[Dan Smith]

Please see the following JVM spec documents:

[1] A minor update to the previously-shared document—changes detailed below.
[2] An adjustment to the Value Classes spec introducing CONSTANT_ClassType to represent direct value class types.
[3] An adjustment to the Value Classes spec to allow value classes to be declared explicitly, rather than derived.

Changes to [1]:
- Allowed vbox to trigger initialization of the VCC.
- Reordered runtime exceptions of vunbox (NPE, not ICCE, if the input is null).
- Added some minor changes to account for the broader domain of CONSTANT_Class strings in 4.7.3, 4.7.5, 4.10.1.3, and 'new'.

On my TODO list:
- Investigate exactly which steps should be taken when loading a $Value class triggers loading of a VCC
- Clean up verification rules to enforce the 4.9.1 static constraints (and maybe move some format checking/static constraints to resolution time?)

[2] and [3] are not necessarily part of MVT, but are natural next steps that we have discussed recently. [2] sets us up for introducing as many type structures as we want and moving away from using CONSTANT_Class to represent types (while continuing to provide legacy support). [3] seems the best path towards dropping the "$Value" convention and, ultimately, having a Q/L pair of types pointing to a single class name. Not sure whether [3] is a viable stopping point for tools and APIs, though, without taking another step to add support for boxed L types.

—Dan

[1] http://cr.openjdk.java.net/~dlsmith/values.html
[2] http://cr.openjdk.java.net/~dlsmith/values-classtype.html
[3] http://cr.openjdk.java.net/~dlsmith/values-declaration.html