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/chaingen.cpp

@@ -458,6 +458,7 @@ bool fill_tx_sources(std::vector<tx_source_entry>& sources, const std::vector<te
               continue;
 
             ts.real_output = realOutput;
+            ts.rct = false;
 
             sources.push_back(ts);