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/integer_overflow.cpp

@@ -63,6 +63,7 @@ namespace
     se.amount = tx.vout[out_idx].amount;
     se.push_output(0, boost::get<cryptonote::txout_to_key>(tx.vout[out_idx].target).key, se.amount);
     se.real_output = 0;
+    se.rct = false;
     se.real_out_tx_key = get_tx_pub_key_from_extra(tx);
     se.real_output_in_tx_index = out_idx;