Mpc support for generic class unions

[thorgeirk11]

Mpc now supports generating formatters base on generic class unions.

• Added CollectGenericUnion method, which reads the union attributes on generic class definitions.
• Formatter names are now created by using the MinimallyQualifiedFormat to support generic classes.

[MessagePackObject]
[Union(0, typeof(Wrapper<string>))]
[Union(1, typeof(Wrapper<int[]>))]
[Union(2, typeof(Wrapper<IEnumerable<Guid>>))]
public class Wrapper<T>
{
    [Key(0)]
    public T Content { get; set; }
}

Mpc will now recognize these Unions and generate the formatters accordingly

...
    case 1: return new MessagePack.Formatters.Wrapper_string_Formatter();
    case 2: return new MessagePack.Formatters.Wrapper_intArray1_Formatter();
    case 3: return new MessagePack.Formatters.Wrapper_IEnumerable_Guid__Formatter();
...

Issue

This enables #964

[neuecc]

Ok to add CollectGenericUnion
but around GetMiniallyQualifiedClassName line, it has some removed code.
I don't know removed code cause what sideeffect.
Can you explain more info?
I worry about the possibility of breaking the existing code.

[thorgeirk11]

The template string was used to generate the formatter classes with a generic type definition.
I found a bug in that approach, e.g.

[MessagePackObject]
public class TreeNode<T> 
{
    [Key(0)]
    public TreeNode<T> Parent { get; set; }   
}

[MessagePackObject]
public class Leaf : TreeNode<Leaf> { }

[MessagePackObject]
public class Leaf2 : TreeNode<Leaf2> { }

When you use mpc on this code it will generate the following:

public sealed class TreeNodeFormatter<T> : IMessagePackFormatter<TreeNode<Leaf>>
{  ...  }
public sealed class TreeNodeFormatter<T> : IMessagePackFormatter<TreeNode<Leaf2>>
{  ...  }

There will be two formatters created with the name TreeNodeFormatter<T> causing a compile error. The new GetMiniallyQualifiedClassName fixes the issue by explicitly stating the type names in the name.

public sealed class TreeNode_Leaf_Formatter : IMessagePackFormatter<TreeNode<Leaf>>
{  ...  }
public sealed class TreeNode_Leaf2_Formatter : IMessagePackFormatter<TreeNode<Leaf2>>
{  ...  }

@neuecc I am not 100% that this will not break anything, what do you think?

[neuecc]

thank you for the clarification.
okay to merge.