Reworked tuple equality

- IFdbTuple inherits from IStructuralEquatable
- FdbTupleComparisons.Default (structural+similar), Bcl and Binary
- Added SimilarValueComparer.Default ("123" = 123 = 123.0)
- IFdbTuple also inherits from IReadOnlyList<object> on .NET 4.5
- Added unit tests
- Why is it so difficult to properly implement equality in .NET ??