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/Makefile.test.include

@@ -57,6 +57,7 @@ BITCOIN_TESTS =\
   test/dbwrapper_tests.cpp \
   test/main_tests.cpp \
   test/mempool_tests.cpp \
+  test/merkle_tests.cpp \
   test/miner_tests.cpp \
   test/mruset_tests.cpp \
   test/multisig_tests.cpp \