Make RandomizedTesting into a separate library

We have ported the generators from RandomizedTesting that Lucene.NET needs to support its tests and added this code to the TestFramework (unlike Lucene), both as static utility classes and as extension methods. The generators we have now in the TestFramework are probably enough to cover most random testing use cases for end users.

That said, it would be useful for the .NET community at large if the random generators were made into a separate library so randomized test data can be easily generated without referencing the Lucene.NET test framework. The latest version of RandomizedTesting has more options for generating random data than what we have ported.

The library should be named after RandomizedTesting (possibly RandomizedTesting.Generators) so that someday if the test runner is also ported (see LUCENENET-627), there is a logical way to integrate it.

There is no reason I can think of to do this before Lucene.NET 4.8.0 is released. This could easily be done out of band with the release and we could integrate into the test framework when Lucene.NET is upgraded to the next version of Lucene.

This task would make a great little project for anyone who wants to contribute to the Lucene.NET effort.