Create Signer interface and use it

types/index.d.ts

@@ -11,7 +11,7 @@ export { OPS as opcodes } from './script';
 export { Transaction } from './transaction';
 export { TransactionBuilder } from './transaction_builder';
 export { BIP32Interface } from 'bip32';
-export { ECPairInterface } from './ecpair';
+export { ECPairInterface, Signer, SignerAsync } from './ecpair';
 export { Network } from './networks';
 export { Payment, PaymentOpts, Stack, StackElement } from './payments';
 export { OpCode } from './script';