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/functional_tests/transactions_generation_from_blockchain.cpp

@@ -113,6 +113,7 @@ bool make_tx(blockchain_storage& bch)
     src.real_out_tx_key = td.m_tx.tx_pub_key;
     src.real_output = interted_it - src.outputs.begin();
     src.real_output_in_tx_index = td.m_internal_output_index;
+    src.rct = false;
     ++i;
   }