Flesh out signInput interface

src/psbt.js

@@ -5,5 +5,17 @@ class Psbt extends bip174_1.Psbt {
   constructor() {
     super();
   }
+  signInput(inputIndex, keyPair) {
+    // TODO: Implement BIP174 pre-sign checks:
+    // https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki#signer
+    // TODO: Get hash to sign
+    const hash = Buffer.alloc(32);
+    const partialSig = {
+      pubkey: keyPair.publicKey,
+      signature: keyPair.sign(hash),
+    };
+    this.addPartialSigToInput(inputIndex, partialSig);
+    return this;
+  }
 }
 exports.Psbt = Psbt;