bug#68244: hash-table improvements

[Mattias Engdegård]

> I don't know in which sense what I propose is different from what you
> mean by "accept both obarray objects and plain vectors": what I propose
> is a way to "accept both obarray objects and plain vectors".

Of course, sorry about that.

And you are right about the need for vector compatibility, just discovered that 
the hard way. The practice to use `make-vector` to create obarrays is 
widespread. (I have patch for the Emacs tree.)