Minimize ‘equal’ calls in (delete x vector)

Minimize ‘equal’ calls in (delete x vector)

* src/fns.c (Fdelete): When deleting from a vector, call Fequal
only once per vector element.  This is faster when Fequal is slow,
and avoids the need to preinitialize the vector result.  Finish
when the result is exhausted, not when the input is exhausted;
the two are equivalent but the former may be faster.
* test/src/fns-tests.el (test-vector-delete): New test.