Add macros for looping through a List without a ListCell.

Many foreach loops only use the ListCell pointer to retrieve the
content of the cell, like so:

    ListCell   *lc;

    foreach(lc, mylist)
    {
        int         myint = lfirst_int(lc);

        ...
    }

This commit adds a few convenience macros that automatically
declare the loop variable and retrieve the current cell's contents.
This allows us to rewrite the previous loop like this:

    foreach_int(myint, mylist)
    {
        ...
    }

This commit also adjusts a few existing loops in order to add
coverage for the new/adjusted macros.  There is presently no plan
to bulk update all foreach loops, as that could introduce a
significant amount of back-patching pain.  Instead, these macros
are primarily intended for use in new code.

Author: Jelte Fennema-Nio
Reviewed-by: David Rowley, Alvaro Herrera, Vignesh C, Tom Lane
Discussion: https://postgr.es/m/CAGECzQSwXKnxGwW1_Q5JE%2B8Ja20kyAbhBHO04vVrQsLcDciwXA%40mail.gmail.com

src/backend/replication/logical/relation.c

@@ -746,11 +746,9 @@ static Oid
 FindUsableIndexForReplicaIdentityFull(Relation localrel, AttrMap *attrmap)
 {
 	List	   *idxlist = RelationGetIndexList(localrel);
-	ListCell   *lc;
 
-	foreach(lc, idxlist)
+	foreach_oid(idxoid, idxlist)
 	{
-		Oid			idxoid = lfirst_oid(lc);
 		bool		isUsableIdx;
 		Relation	idxRel;
 		IndexInfo  *idxInfo;

src/backend/replication/logical/tablesync.c

@@ -1036,11 +1036,11 @@ fetch_remote_table_info(char *nspname, char *relname,
 
 		/* Build the pubname list. */
 		initStringInfo(&pub_names);
-		foreach(lc, MySubscription->publications)
+		foreach_node(String, pubstr, MySubscription->publications)
 		{
-			char	   *pubname = strVal(lfirst(lc));
+			char	   *pubname = strVal(pubstr);
 
-			if (foreach_current_index(lc) > 0)
+			if (foreach_current_index(pubstr) > 0)
 				appendStringInfoString(&pub_names, ", ");
 
 			appendStringInfoString(&pub_names, quote_literal_cstr(pubname));