New "Halfway RingCT" outputs for coinbase transactions

When RingCT is enabled, outputs from coinbase transactions
are created as a single output, and stored as RingCT output,
with a fake mask. Their amount is not hidden on the blockchain
itself, but they are then able to be used as fake inputs in
a RingCT ring. Since the output amounts are hidden, their
"dustiness" is not an obstacle anymore to mixing, and this
makes the coinbase transactions a lot smaller, as well as
helping the TXO set to grow more slowly.

Also add a new "Null" type of rct signature, which decreases
the size required when no signatures are to be stored, as
in a coinbase tx.

tests/core_tests/rct.h

@@ -81,7 +81,7 @@ private:
 
 template<>
 struct get_test_options<gen_rct_tx_validation_base> {
-  const std::pair<uint8_t, uint64_t> hard_forks[2] = {std::make_pair(1, 0), std::make_pair(4, 1)};
+  const std::pair<uint8_t, uint64_t> hard_forks[3] = {std::make_pair(1, 0), std::make_pair(2, 1), std::make_pair(4, 65)};
   const cryptonote::test_options test_options = {
     hard_forks
   };