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/hashtab.c b/src/hashtab.c
index 54f7500..ad01857 100644
--- a/src/hashtab.c
+++ b/src/hashtab.c
@@ -51,7 +51,7 @@
 {
     hashtab_T *ht;
 
-    ht = (hashtab_T *)alloc(sizeof(hashtab_T));
+    ht = ALLOC_ONE(hashtab_T);
     if (ht != NULL)
 	hash_init(ht);
     return ht;
@@ -400,7 +400,7 @@
     else
     {
 	/* Allocate an array. */
-	newarray = (hashitem_T *)alloc(sizeof(hashitem_T) * newsize);
+	newarray = ALLOC_MULT(hashitem_T, newsize);
 	if (newarray == NULL)
 	{
 	    /* Out of memory.  When there are NULL items still return OK.