Constant Descriptors usage and Latest versions

[Brian Goetz]

> For 2, this problem is actually more related to the Constant API, for
> Constant Descriptors themselves are prime candidates to be Constable,
> as they are already immutable.

Fun fact: in the initial prototype they were Constable, at which point 
we realized that some work was needed in the condy protocol to make it 
practice for libraries to do this, so we backed out the `implements 
Constable` "temporarily" while waiting for some work on condy.  Which 
has been a while at this point.  (The main issue is that many condy's 
would like to use an ordinary static method for their bootstrap, rather 
than libraries having to have two versions of each factory, one for 
bytecoded use and one for condy use, but condy bootstraps currently 
require the three metadata arguments, even when not used.)