Inline signature serializer

Instead of building a full copy of a CTransaction being signed, and
then modifying bits and pieces until its fits the form necessary
for computing the signature hash, use a wrapper serializer that
only serializes the necessary bits on-the-fly.

This makes it easier to see which data is actually being hash,
reduces load on the heap, and also marginally improves performances
(around 3-4us/sigcheck here). The performance improvements are much
larger for large transactions, though.

The old implementation of SignatureHash is moved to a unit tests,
to test whether the old and new algorithm result in the same value
for randomly-constructed transactions.

src/test/Makefile.am

@@ -34,7 +34,7 @@ test_bitcoin_SOURCES = accounting_tests.cpp alert_tests.cpp \
   netbase_tests.cpp pmt_tests.cpp rpc_tests.cpp script_P2SH_tests.cpp \
   script_tests.cpp serialize_tests.cpp sigopcount_tests.cpp test_bitcoin.cpp \
   transaction_tests.cpp uint160_tests.cpp uint256_tests.cpp util_tests.cpp \
-  wallet_tests.cpp $(JSON_TEST_FILES) $(RAW_TEST_FILES)
+  wallet_tests.cpp sighash_tests.cpp $(JSON_TEST_FILES) $(RAW_TEST_FILES)
 
 nodist_test_bitcoin_SOURCES = $(BUILT_SOURCES)