Add obarray-clear and use it

Add obarray-clear and use it

* lisp/obarray.el (obarray-clear): New.
* lisp/abbrev.el (clear-abbrev-table):
* lisp/vc/vc.el (vc-clear-context): Use it instead of assuming the
obarray is a vector that can be 0-filled.
* test/lisp/obarray-tests.el (obarray-clear): New test.

(cherry picked from commit 6a182658a533acab94d8fa0aec3e2b7a4f7d6a93)