0.4.0

Added

• Wallet which allows users to select coins through a backend configured by them via the GetUtxos trait.
  The provided wallet comes with a UTXO cache which this is updated using Wallet::sync.
  This allows users of the library to optimise the number of requests to their backend.
  Users can also sign said UTXOs by calling Wallet::sign.
• Timelock type to allow users of the library to explicitly choose the type of timelock they want to use when building the loan transaction.
  For the time being, users can still pass in a u32, but they are encouraged not to.

Changed

• The borrower can now choose the collateral inputs before calling Borrower0::new.

Fixed

• The loan transaction no longer expects collateral and principal change outputs.
  We were unconditionally adding them to the transaction which could result in attempting to build a 0-valued confidential transaction output, which is invalid.**