Internal enums trip up StandardResolverAllowPrivate

[AArnott]

The StandardResolverAllowPrivate resolver fails on non-public enum types:

        internal enum InternalEnum
        {
            One,
            Two,
        }

        [MessagePackObject]
        internal class InternalClass
        {
            [Key(0)]
            internal InternalEnum EnumProperty { get; set; }
        }


        [Fact]
        public void InternalClassWithInternalEnum()
        {
            InternalClass expected = new InternalClass
            {
                EnumProperty = InternalEnum.Two,
            };

            byte[] bytes = MessagePackSerializer.Serialize(expected, StandardResolverAllowPrivate.Options);
            InternalClass actual = MessagePackSerializer.Deserialize<InternalClass>(bytes, StandardResolverAllowPrivate.Options);
            Assert.Equal(expected.EnumProperty, actual.EnumProperty);
        }

Message: 
    MessagePack.MessagePackSerializationException : Failed to serialize MessagePack.Tests.AllowPrivateTest+InternalClass value.
    ---- System.TypeInitializationException : The type initializer for 'FormatterCache`1' threw an exception.
    -------- System.TypeLoadException : Type 'MessagePack.Formatters.MessagePack_Tests_AllowPrivateTest\+InternalEnumFormatter1' from assembly 'MessagePack.Resolvers.Dynamicp a mesEnumResolver, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is attempting to implement an inaccessible interface.

[AArnott]

@neuecc Do you have any thoughts on this? Do we need to create another dynamic method system to handle this like we have for non-public classes/structs?

[neuecc]

@AArnott

The implementation is already in GenericEnumFormatter,
and besides, this is included in the DynamicGenericResolver, so remove DynamicEnumResolver from StandardResolverHelper, it will work.

• Erase DynamicEnumResolver only AllowPrivate
• Erase in both cases

There was no Unsafe before.
We also thought it would be advantageous in terms of performance because it would not pass through delegates.
However, I think it's a good to use only GenericEnumFormatter.

[AArnott]

Awesome. I'm so glad I asked because I was about to go to great lengths to fix it in the wrong way. Your way was deleting one line. :)