Adding filter and skipIf methods with an entry predicate for BiStream class

[d-william]

Similar to methods filter and skipIf with bi predicate

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[fluentfuture]

If you have a BiStream and would like to treat it as a Stream<Entry>, consider to call .mapToObj(Map::entry) ?

The main motivation behind BiStream is that you don't have to deal with entries.

Or can you clarify the use case where existing filter() aren't sufficient?