Call ImmutableList<T>.AddRange from ConvertAll and FindAll

There are 2 more places where we can benefit from the performance optimization in ImmutableList<T>.AddRange

My benchmarking shows the following results:

    Benchmarking FindAll:
    Using random seed -1979800607
    Optimized; iterations: 20; Time: 00:00:00.3338650
    Original; iterations: 20; Time: 00:00:00.7364930

    Benchmarking ConvertAll:
    Using random seed -1967300607
    Optimized; iterations: 20; Time: 00:00:00.3541609
    Original; iterations: 20; Time: 00:00:01.2586320

(Benchmark code is here: https://github.com/PatrickMcDonald/corefx/tree/immutablelist-addrange-refactor-perf)

I'd rather not import this namespace because we always need to be cautious when using Linq from this library because of its perf implications. In this case, your use of it is good, but we want that to be an explicit intention. Can you please remove this `using` directive and instead, each time you want to invoke a linq extension method use the full method syntax?
For example:

    return root.AddRange(this.Select(converter));

becomes (I think)

    return root.AddRange(Enumerable.Select(this, converter));