Value-Based Classes

This is my shot at implementing [Feature Request #313: Warn about identity-sensitive operations on value based classes ](https://sourceforge.net/p/findbugs/feature-requests/313/).

For brevity's sake I'll use _vbi_ for _value-based instance(s)_, i.e. an instance of a value-based class.

## Current State:

* :mag: no reference comparison
* :white_large_square: serialization of _vbi_
    * :mag: no non-transient field in serializable class
    * :white_large_square: no direct serialization via `ObjectOutputStream.writeObject`
* :white_large_square: identity hash code
    * :white_large_square: no argument to `System.identityHashCode`
    * :white_large_square: no key in an `IdentityHashMap`
* :white_large_square: no locking on a _vbi_