Additionally, this change removes ‘struct TALER_PaymentResponsePS’
from the public API.
* src/include/taler_crypto_lib.h
(TALER_merchant_pay_sign): New func decl.
(TALER_merchant_pay_verify): New func decl.
* src/include/taler_signatures.h
(struct TALER_PaymentResponsePS): Delete.
* src/util/merchant_signatures.c
(struct TALER_PaymentResponsePS): Move here from taler_signatures.h.
(TALER_merchant_pay_sign): New func.
(TALER_merchant_pay_verify): New func.
