Switch blocks to a constant-space Merkle root/branch algorithm.

This switches the Merkle tree logic for blocks to one that runs in constant (small) space.
The old code is moved to tests, and a new test is added that for various combinations of
block sizes, transaction positions to compute a branch for, and mutations:
 * Verifies that the old code and new code agree for the Merkle root.
 * Verifies that the old code and new code agree for the Merkle branch.
 * Verifies that the computed Merkle branch is valid.
 * Verifies that mutations don't change the Merkle root.
 * Verifies that mutations are correctly detected.

src/test/main_tests.cpp

@@ -72,5 +72,4 @@ BOOST_AUTO_TEST_CASE(test_combiner_all)
     Test.disconnect(&ReturnTrue);
     BOOST_CHECK(Test());
 }
-
 BOOST_AUTO_TEST_SUITE_END()