fix add on dataview missing byteOffset from previous dv

shim.ts

@@ -1,5 +1,5 @@
 DataView.prototype.add = function (offset) {
-  return new DataView(this.buffer, offset);
+  return new DataView(this.buffer, offset + this.byteOffset);
 };
 DataView.prototype.writeU8 = function (val) {
   return this.setUint8(0, val);

tests/fn.test.js

@@ -106,6 +106,13 @@ describe("module", () => {
     XqBytesEnc(in_buf, long_dec_bytes.byteLength, 4); // this mutates in_buf
     assert.deepEqual(new Uint8Array(in_buf.buffer), long_enc_bytes);
   });
+  /* TODO
+  it("decrypts offset dataviews", () => {
+    const in_buf = new DataView(simple_enc_bytes.buffer.slice(0));
+    XqBytesDec(in_buf, simple_enc_bytes.byteLength, 4); // this mutates in_buf
+    assert.deepEqual(new Uint8Array(in_buf.buffer), simple_dec_bytes);
+  });
+  */
   it("reverts Enc with Dec", () => {
     const in_buf = new DataView(long_dec_bytes.buffer.slice(0));
     XqBytesEnc(in_buf, long_dec_bytes.byteLength, 4); // this mutates in_buf