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/diff.c b/src/diff.c
index 0db2820..260fdd9 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -537,7 +537,7 @@
 {
     diff_T	*dnew;
 
-    dnew = (diff_T *)alloc(sizeof(diff_T));
+    dnew = ALLOC_ONE(diff_T);
     if (dnew != NULL)
     {
 	dnew->df_next = dp;