Add a string equality check function to string pool API

- As a special case, two strings (ids) from the same pool can be tested for
  equality in constant time (integer comparison). If the pools differ,
  a regular string comparison is needed.

rpmio/rpmstrpool.c

@@ -173,3 +173,12 @@ size_t rpmstrPoolStrlen(rpmstrPool pool, rpmsid sid)
     }
     return slen;
 }
+
+int rpmstrPoolStreq(rpmstrPool poolA, rpmsid sidA,
+		    rpmstrPool poolB, rpmsid sidB)
+{
+    if (poolA == poolB)
+	return (sidA == sidB);
+    else
+	return rstreq(rpmstrPoolStr(poolA, sidA), rpmstrPoolStr(poolB, sidB));
+}

rpmio/rpmstrpool.h

@@ -36,6 +36,10 @@ const char * rpmstrPoolStr(rpmstrPool sidpool, rpmsid sid);
 /* get a strings length by its id (in constant time) */
 size_t rpmstrPoolStrlen(rpmstrPool pool, rpmsid sid);
 
+/* pool string equality comparison (constant time if within same pool) */
+int rpmstrPoolStreq(rpmstrPool poolA, rpmsid sidA,
+                    rpmstrPool poolB, rpmsid sidB);
+
 #ifdef __cplusplus
 }
 #endif