Add ExtractToImmutable

This is an API which allows for a zero-copy creation of an ImmutableArray<T> value.  

This is an alternative approach to the PR #154 

@jaredpar, both of those examples throw ArgumentOutOfRangeException, for both the lower and upper bound, so if the goal is to match those, seems like the wrong exception is in use.  But, I get the point, it's from prior to your change, and it's not a big deal, so ignore me :)