Convertible types need public default contructor

[ecorm]

Variant::to<CustomType>(CustomType& dest) current requires that CustomType be default-constructible. Try to make it so CustomType does not require a default constructor, or, at least, the public contructor can be private and access be granted via wamp::ConversionAccess.

For Variant::to<CustomType>(), make it so the public constructor can be private and access be granted via wamp::ConversionAccess.