opt: Skip evaluation of equivalent input sets

When two successive UTXOs match in effective value and weight, we can
skip the second if the prior is not selected: adding it would create an
equivalent input set to a previously evaluated.

E.g. if we have three UTXOs with effective values {5, 3, 3} of the same
weight each, we want to evaluate
{5, _, _}, {5, 3, _}, {5, 3, 3}, {_, 3, _}, {_, 3, 3},
but skip {5, _, 3}, and {_, _, 3}, because the first 3 is not selected,
and we therefore do not need to evaluate the second 3 at the same
position in the input set.

If we reach the end of the branch, we must SHIFT the previously selected
UTXO group instead.

src/wallet/test/coinselector_tests.cpp

@@ -1180,7 +1180,7 @@ BOOST_AUTO_TEST_CASE(coin_grinder_tests)
         });
         BOOST_CHECK(res);
         // Demonstrate how following improvements reduce iteration count and catch any regressions in the future.
-        size_t expected_attempts = 100'000;
+        size_t expected_attempts = 184;
         BOOST_CHECK_MESSAGE(res->GetSelectionsEvaluated() == expected_attempts, strprintf("Expected %i attempts, but got %i", expected_attempts, res->GetSelectionsEvaluated()));
     }
 
@@ -1202,7 +1202,7 @@ BOOST_AUTO_TEST_CASE(coin_grinder_tests)
         add_coin(1 * COIN, 2, expected_result);
         BOOST_CHECK(EquivalentResult(expected_result, *res));
         // Demonstrate how following improvements reduce iteration count and catch any regressions in the future.
-        size_t expected_attempts = 4;
+        size_t expected_attempts = 3;
         BOOST_CHECK_MESSAGE(res->GetSelectionsEvaluated() == expected_attempts, strprintf("Expected %i attempts, but got %i", expected_attempts, res->GetSelectionsEvaluated()));
     }
 
@@ -1271,7 +1271,7 @@ BOOST_AUTO_TEST_CASE(coin_grinder_tests)
         BOOST_CHECK(EquivalentResult(expected_result, *res));
         // Demonstrate how following improvements reduce iteration count and catch any regressions in the future.
         // If this takes more attempts, the implementation has regressed
-        size_t expected_attempts = 82'407;
+        size_t expected_attempts = 43;
         BOOST_CHECK_MESSAGE(res->GetSelectionsEvaluated() == expected_attempts, strprintf("Expected %i attempts, but got %i", expected_attempts, res->GetSelectionsEvaluated()));
     }