Improve invocation support

[TheConstructor]

Currently you can't use Expression.Invoke with a delegate as first argument, as LINQKit expects that you only invoke other lambdas. While that may be enough for EF Core, there are other things, that you can do with LINQKit.

Also I added support for .Compile().Invoke() and not just .Compile()().

[TheConstructor]

Hey @StefH would you happen to have time to review this? This would allow to use LINQKit for some more use-cases outside of EF Core

[StefH]

I need to find time for this...