finance/taler-exchange: (new port) distributed privacy-preserving payment system

GNU Taler is a privacy-preserving payment system. Customers can stay
anonymous, but merchants can not hide their income through payments
with GNU Taler. This helps to avoid tax evasion and money laundering.

The primary use case of GNU Taler is payments; it is not meant as a
store of value. Payments are always backed by an existing currency.

Payments are made after exchanging existing money into electronic
money with the help of an Exchange service, that is, a payment
service provider for Taler.

When making a payment, customers only need a loaded wallet. A
merchant can accept payments without making their customers register
on the merchant's Website.

Supported-by: NLNet
Supported-by: Commons Caretakers