Caching lambda proxy classes

[Brian Goetz]

> A question: What's so special in the $deserializeLambda$ method that it
> has to be generated in each capturing class? The MethodHandles.Lookup?

Security.  Anyone can create a serialized bytestream that effectively 
says "take this private method of a random class, and turn it into a 
lambda."  We wish to only proceed with lambda conversion for 
combinations of (SAM, method) that *actually* were intended to be lambdas.