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/performance_tests/multi_tx_test_base.h

@@ -72,6 +72,7 @@ public:
     source_entry.real_output_in_tx_index = 0;
     source_entry.outputs.swap(output_entries);
     source_entry.real_output = real_source_idx;
+    source_entry.rct = false;
 
     m_sources.push_back(source_entry);