patch 8.1.1414: alloc() returning "char_u *" causes a lot of type casts

Problem:    Alloc() returning "char_u *" causes a lot of type casts.
Solution:   Have it return "void *". (Mike Williams)  Define ALLOC_ONE() to
            check the simple allocations.
diff --git a/src/sign.c b/src/sign.c
index 9766d4e..b89bea0 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -85,7 +85,7 @@
     if (HASHITEM_EMPTY(hi))
     {
 	// new group
-	group = (signgroup_T *)alloc(sizeof(signgroup_T) + STRLEN(groupname));
+	group = alloc(sizeof(signgroup_T) + STRLEN(groupname));
 	if (group == NULL)
 	    return NULL;
 	STRCPY(group->sg_name, groupname);
@@ -201,8 +201,7 @@
 {
     signlist_T	*newsign;
 
-    newsign = (signlist_T *)lalloc_id(sizeof(signlist_T), FALSE,
-							      aid_insert_sign);
+    newsign = lalloc_id(sizeof(signlist_T), FALSE, aid_insert_sign);
     if (newsign != NULL)
     {
 	newsign->id = id;
@@ -736,7 +735,7 @@
     int	start = next_sign_typenr;
 
     // Allocate a new sign.
-    sp = (sign_T *)alloc_clear_id(sizeof(sign_T), aid_sign_define_by_name);
+    sp = alloc_clear_id(sizeof(sign_T), aid_sign_define_by_name);
     if (sp == NULL)
 	return NULL;