There are quite a few variables in the code that are no longer
(or perhaps never were) in use. These were discovered by enabling
compiler warnings for unused variables and cleaning them up.
In most cases where the unused variables were the result
of a function call the call was left but the variable
assignment removed, unless it was obvious that it was
a simple getter with no side effects.
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.
