Tutorial: spellcasting

Add some basic spellcasting stuff to the tutorial: read a spellbook,
cast a spell.  If the hero doesn't have enough energy, just adds
a note saying so.

Remove/restore the known spells when entering/leaving the tutorial.

src/decl.c

@@ -372,6 +372,7 @@ static const struct instance_globals_g g_init_g = {
     0L, /* gmst_moves */
     NULL, /* gmst_invent */
     NULL, NULL, NULL, /* gmst_ubak, gmst_disco, gmst_mvitals */
+    { DUMMY }, /* gmst_spl_book */
     /* pline.c */
     UNDEFINED_PTR, /* gamelog */
     /* region.c */