Moves some functionality for creating RingCT transactions into helper
functions.

Removes a duplicate error for when setting a fee of 0 fails. (Later we will
attempt to set a fee of nFeeRet, which would be 0 in this case anyway.)

Change CreateTxOuts to create the actual TxOut pointers
rather than fill a list of TxOuts and separately extract the pointers.

This sets up a basic but incomplete implementation of CreateRingCTStake.

Includes a weighting function that groups coins into buckets along
powers of 16 sats, where the weight of the coin would be equal to the
smallest value in the bucket regardless of the actual value of the coin.

Refactor out the necessary vectors to store separately in the stakeinput,
since we need them from both the input blinds and the output blinds.

And allow static allocation of the context structs.

Move COutputR to a common location.
Hold a copy of COutputR in RingCTStake for its lifetime.
Handle other memory lifetime issues.

Add a helper class for resetting pending spends.

and unmark pending spends if the block is not created.

- Provide the txCoinbase to the CoinStake object for filling out
further if necessary. (Originally, so that we could reference them in
the coinstake, but this won't be necessary if we make them ringct to
begin with. Still, it's easier to have in one place.)
- Fix two misc error() calls that were broken.
- Use blind_sum and verify_tally to confirm that the anon coin rewards
  sum to the expected number.

Remove some debugging statements and other cleanup.

- Stake uniqueness is the hash of the public key of the input.
- Stake indexfrom is the block index at the minimum stake depth.

As the stake weight controls the hash difficulty ceiling, we can simply
choose the lowest bracket in which the stake hash comes under the
ceiling, and use that as our rangeproof min_value for added privacy.

- Fix overflow detection when multiplying arith_uint256.
  Otherwise regtest stake targets can be a little wonky.
- Remove an unnecessary back-and-forth cast to uint256.
- The RingCT stake bracket weights are basically constants, so do that.

Add enable_wallet guards to similar places as existing ones.
Remove unused stake functions from early attempts.

And add include.