Add marker types

[vbfox]

When a generic method need to be called a type need to be provided even if we don't care about it:

type.Annotate<T>(l => l.Debug(CanBeNull<T>()));

While the library should work with any marker type user provided (To match complex constraints) it should also contain types for the most common cases (class and struct) :

class T
{
}

struct TStruct
{
}

[vbfox]

All is now working.