Add Flatten to monads

[csillikd-messerli]

I deliberately chose Flatten; I find Join less self-explanatory.

[csillikd-messerli]

@bash @FreeApophis Do you have any idea about the error? Lazy also has a public parameterless ctor!

[bash]

Yeah Lazy is a bit annoying because its generic type is annotated with [DynamicallyAccessedMembers(PublicParameterlessConstructor)] and this leaks out in the sense that you have to annotate all your Ts with that attribute too.

I think defining your own Identity function should do the trick:

[Pure]
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static T Identity<[DynamicallyAccessedMembers(PublicParameterlessConstructor)] T>(T value) => value;

You can see how I dealt with this issue here: https://github.com/polyadic/funcky/blob/main/Funcky/Monads/Lazy/Lazy.Monad.cs 😄