*

* Returns the key for the first-found entry with the given +value+

* (see {Entry Order}[rdoc-ref:Hash@Entry+Order]):

* h = {foo: 0, bar: 2, baz: 2}

* h.key(0) # => :foo

* h.key(2) # => :bar

*

* Returns +nil+ if no such value is found.

*/

static VALUE