palindrome-products: optimise implementation

[SleeplessByte]

Don't work on this issue until #895 is merged, or use that PR as a base branch.

The "largest palindrome from four digit factors" test currently takes > 30s to complete. When fixed, replace test.skip with xtest.

[tliebert]

This test case currently prevents the users code from passing the test suite - it returns this case as failed even though the test is skipped.

[junedev]

@tliebert Thanks for reporting. We are aware of the issue. @SleeplessByte is working on fixing this.