Weak hash table support

[ehuelsmann]

In order to allow ​trivial-garbage to implement support for ABCL, we need to provide weak hash table support.

[Mark Evenson]

Is there some obvious reason why a wrapping of the built-in Java constructs of ​WeakHashMap and ​WeakReference wouldn't suffice here?

[ehuelsmann]

Yes: Our own hashes support concurrent readers and a single writer. That's better than WeakHashMap?, which doesn't support concurrency at all. Also, trivial-garbage would like to provide support for hashes which allow for weak keys as well as weak values.

At the same time, our own hashes support EQ, EQL, EQUAL and EQUALP hash equality. BTW: The fact that this ticket exists doesn't mean it will take ages to implement: we probably need to copy the file which currently defines our hashes, make the bucket class abstract and define accessors to the keys and values - instead of accessing them directly as our hash source does now.

[Mark Evenson]

Unsure about concurrent readers in the current implementation: wouldn't the EQUALP method on a HashTable? would fail if its first argument was being modified?

[Mark Evenson]

Completed with r13309 and r13310.