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/transaction_tests.cpp

@@ -97,6 +97,7 @@ bool test_transaction_generation_and_ring_signature()
 
     src.real_out_tx_key = cryptonote::get_tx_pub_key_from_extra(tx_mine_2);
     src.real_output = 1;
+    src.rct = false;
     src.real_output_in_tx_index = 0;
   }
   //fill outputs entry