Use union for the payload of struct Lisp_Vector.

Use union for the payload of struct Lisp_Vector.
This helps to avoid a few glitches dictated by C's aliasing rules.
* lisp.h (struct Lisp_Vector): Use union for next and
contents member.  Adjust comment.  Change related users.
* alloc.c (next_in_free_list, set_next_in_free_list): Remove.
Related users changed.
* buffer.c, bytecode.c, ccl.c, character.h, chartab.c, composite.c:
* composite.h, disptab.h, fns.c, fontset.c, indent.c, keyboard.c:
* lread.c, msdos.c, process.c, w32menu.c, window.c, xdisp.c:
* xfaces.c, xfont.c, xmenu.c: Related users changed.