Generalize common compile-time constants.

Generalize common compile-time constants.
* lisp.h (header_size, bool_header_size, word_size): Now here.
(struct Lisp_Vector): Add comment.
(struct Lisp_Bool_Vector): Move up to define handy constants.
(VECSIZE, PSEUDOVECSIZE): Simplify.
(SAFE_ALLOCA_LISP): Use new constant.  Adjust indentation.
* buffer.c, buffer.h, bytecode.c, callint.c, eval.c, fns.c:
* font.c, fontset.c, keyboard.c, keymap.c, macros.c, menu.c:
* msdos.c, w32menu.c, w32term.h, window.c, xdisp.c, xfaces.c:
* xfont.c, xmenu.c: Use word_size where appropriate.