wallet/size: add EstimateVirtualSize
wallet/size: add EstimateVirtualSize

This commit adds a new method EstimateVirtualSize that calculates
the worst case estimate vsize for a transaction with a given set
of inputs and outputs. This method is aware of P2PKH, P2WPKH and
P2SH-P2WPKH inputs, and caulculates the transaction vsize with
the witness data included.

Roasbeef/btcwallet,Roasbeef/btcwallet