[constables] RFR of constants API

[Brian Goetz]

> * I believe the name descriptor is going to be confusing with the rest 
> of the API, as there are a lot of classes whose name ends with 'Desc'

I think this speaks to these classes ({Type,Field,Method}Descriptor) 
being in the wrong place.  I think they should be in JLI, since they are 
basic building blocks for describing VM types.  Also should be 
TypeDescriptor, FieldTypeDescriptor, MethodTypeDescriptor.